Design guide for LED Lighting Controller XC9401(7/9)

7. Selection of the external components of an isolated circuit

This section explains how to select external components for an isolated circuit. An isolated circuit using the XC9401 A type at 220VAC as shown in Fig. 33 is used as an example.

Fig.33 220VAC/240VAC Isolated flyback, A type Typical Application Circuit

7-1. Number of LED Series

The criteria for selecting the number of LED series in this application is as follows.
In an isolated flyback circuit, a flyback voltage proportional to the LED voltage is applied to the external power MOSFET in addition to the AC input (refer to section 7-9). For this reason, if the number of LED series is large, the voltage applied to the external power MOSFET increases, a larger rated voltage must be used, and cost increases or efficiency decreases due to larger on-resistance.

For this reason, in an isolated flyback circuit, it is important to set a small number of LED series and reduce overall cost, including peripheral components.

7-2. Bridge Diode (BR1)

This is a bridge diode for full wave rectification of the AC input. Select a bridge diode with a peak inverse voltage and average rectification current that are more than sufficient for the input voltage and current.
In this example, the peak value of the input current is about 70mA and the maximum voltage applied to the bridge diode is about 620V, and thus a product with a rated current of 0.8 A and a rated voltage of 800V is selected.

7-3. Input Filter (L1,L2,C1,C2,C7)

C1 and L1 form a filter circuit that reduces noise from the AC input and noise that returns to the AC input. In the typical circuit shown in Fig. 33, a filter is formed that attenuates 20kHz and higher noise to remove switching frequency (50kHz to 150kHz) and higher noise. It will be necessary to adjust the input filter constants and filter circuit to meet the regulations and standards that will actually apply. The capacitance value of C1 must be kept small to limit rush current from the AC input, so select a capacitor that is about 0.1μF.

To improve the power factor in this circuit (Fig. 33), a signal in phase with the AC input is input into the VSINE pin. For this reason, if the C2 capacitance value is large, the signal input into the VSINE pin falls out of phase with the AC input and the power factor decreases. Select a capacitance value of about 0.1μF.

C7 is a capacitor that is connected between the primary side and secondary side to reduce the EMI level. Because the primary side and secondary side are isolated from each other, a normal capacitor cannot be inserted. Instead, select a certified capacitor that meets the applicable regulations and safety standards.

7-4. VSINE Pin (R1,R2)

In the A type used in this example, the voltage after full wave rectification is divided by R1 and R2 and applied to the VSINE pin. By comparing the VSINE pin voltage to the ISEN pin voltage that results from converting the current that flows to the external power MOSFET to a voltage with R3 and R4, the current that flows to the external power MOSFET is controlled. (Fig. 34)

Fig. 34 Current to the external power MOSFET and transformer

For the R1 and R2 resistance values, select values that satisfy Equation (15) with the R2 value no more than 10kΩ.

R1、R2 Refer to fig.33.
Vrms_max Maximum input RMS voltage

A calculation example is shown below.
When R2 = 10kΩ with Vrms_max=240V, the resistance value of R1 is

Select a resistance value within this range.
The same calculation is made in the separate calculation file. Please make use of this.

7-5. Power Supply to VDD pin (R5,R6,R9,C3,D3,LT1)

This supplies power to the power pin (VDD pin) of the IC using the auxiliary coil of the transformer.

Fig.35 VDD power supply circuit using a transformer

LT1

Current is supplied to the VDD pin using the LT1 auxiliary coil.
For selection of the transformer, refer to section 7-6.

DVDD

This is a rectifying diode that supplies power voltage from LT1. A reverse bias voltage VDvdd that depends on the LED voltage and transformer turn ratio as shown in Equation (16) is applied to DVDD. Select a diode with a rated voltage appropriate for this reverse bias voltage.

N1 Number of windings of transformer primary coil
NAUX Number of windings of transformer auxiliary coil
VDD VDD pin voltage
Vrms_max Maximum input RMS voltage
Vspike Spike voltage that accompanies switching (to 50 V)

A calculation example is shown below.
When NAUX/ N1=1/6.74, VDD=12V, Vrms_max=240V, Vspike=50V the reverse bias voltage VDvdd is

The same calculation is made in the separate calculation file. Please make use of this file.

CVDD

This capacitor stabilizes the VDD pin voltage. Use a capacitor with a capacitance of 10μF or higher. If a ceramic capacitor will be used, select a product in which the electrostatic capacitance falls minimally when a B type (JIS Standards) or X7R/X5R (EIA Standards) DC bias is applied.

RVDD

This resistance is used to supply power to the VDD pin at startup. When the input voltage is applied and the VDD pin voltage rises above the UVLO release voltage, GATE output starts and normal operation takes place. After startup, power is mainly supplied to the VDD pin through the auxiliary coil of the transformer.

When RVDD is large and the current through RVDD is smaller than the current consumed in the IC, the VDD pin voltage does not rise higher than the UVLO release voltage and startup is not possible. For this reason, select a resistance value for RVDD that satisfies Equation (17). (Fig. 35)

ISTB Stand-by Current 225μA (typ.)
VUVLOR UVLO Release Voltage 7.5V (typ.)
Vrms_min Minimum input RMS voltage

A calculation example is shown below.
When ISTB=225µA, VUVLOR=7.5V, Vrms_min=200V, RVDD is

and the IC can be started normally by using a resistance value lower than 1.22 MΩ.
The same calculation is made in the separate calculation file. Please make use of this.

RVDD1

To supply current to the VDD pin, LX_VDD is made to oscillate and supply voltage to the VDD pin (refer to Fig. 36). However, in actuality a spike voltage sometimes occurs in LX_VDD and causes the VDD pin voltage to rise higher than the VDD target voltage (= VLED × N3/N2). A countermeasure for this VDD pin voltage rise is to insert a resistance in RVDD1 to reduce the current supplied to the VDD pin.

Fig.36 Operation waveforms (VDD power supply circuit using a transformer)

7-6. Transformer (LT1)

This is a transformer in the isolated flyback circuit that transfers electrical energy from the primary side to the secondary side by magnetic coupling. The peripheral transformer circuit is shown in Fig. 37.
For the transformer, either a general purpose transformer or a transformer with custom specifications can be used. Component selection is explained below for each case.

Fig.37 Peripheral circuit of transformer

Fig.38 Transformer current and on time/off time

Selection method for general purpose transformer

Selection of a general purpose transformer is based on whether the output power, turn ratio, and inductance value satisfy the applicable regulations and standards. The method of selecting each parameter is explained below.

Output power

Select a transformer with an output power that has sufficient allowance for the output power of the LED. The amount of loss will vary depending on the operation frequency and other factors, so determine usability by checking the transformer temperature in the actual equipment.
In this example, a transformer with an output power of 12 W is selected for a LED output power of 7W.

Turn ratio

Select a turn ratio for the primary and secondary windings of the transformer (= N1/N2) of about 5 to 10. In general, a larger turn ratio causes a larger leakage inductance, which decreases efficiency and increases the allowable loss of resistor R8 in the snubber circuit. This results in increased cost.

By setting the number of turns of the auxiliary coil that supplies power to the VDD pin to the value calculated from Equation (22), the VDD pin voltage can be set to the target value. Note, however, that if the number of LED series changes, the VDD pin voltage will also change.
In actuality, pike voltages may occur and cause the VDD pin voltage to occasionally rise higher than the VDD voltage target value. Refer to section 7-5 for the countermeasure for this.

Inductance value

This application is controlled to operate normally in discontinuous mode. In continuous mode, operation may become unstable. For this reason, select an inductance value for the transformer primary coil that keeps operation in discontinuous mode.

First, calculate the maximum inductance required to enter discontinuous mode from Equation (18). Select an inductance value for the primary coil that is smaller than the maximum inductance value. As a general guideline, select an inductance value that gives a oscillation frequency of about 100 kHz.

The maximum voltage applied to the MOSFET, rectifying diode, snubber circuit, and other peripheral components depends on the turn ratio (N (= N1/N2) of the primary coil and secondary coil. For this reason, the optimum rated voltage of the peripheral components varies depending on the turn ratio. Select the turn ratio to optimize the overall cost, including peripheral components.

N Turn ratio of transformer primary coil and secondary coil (=N1/N2)
VF Forward voltage of rectifying diode
VLED LED voltage
tOFF Off time 6.0μs(typ.)
IL1_max Maximum value of current in primary coil
Applicable regulations and safety standards

Conduct testing to verify whether the transformer selected above can meet the applicable regulations and standards.

Procedure for designing a custom transformer

A procedure for designing a transformer with custom specifications is given as an example below. In actual practice, consult with the transformer manufacturer prior to considering and developing transformer specifications.
The transformer characteristics may deviate from the design values due to leakage inductance, the coil winding method, and other factors. Test in the actual equipment before selecting the transformer.

Inductance value and turn ratio of primary and secondary coils

This application is controlled to operate normally in discontinuous mode. In continuous mode, operation may become unstable. For this reason, select an inductance value for the transformer primary coil that keeps operation in discontinuous mode.

First, calculate the maximum inductance required to enter discontinuous mode from Equation (18). Select an inductance value for the primary coil that is smaller than the maximum inductance value. As a general guideline, select an inductance value that gives a oscillation frequency of about 100 kHz.

The maximum voltage applied to the MOSFET, rectifying diode, snubber circuit, and other peripheral components depends on the turn ratio (N (= N1/N2) of the primary coil and secondary coil. For this reason, the optimum rated voltage of the peripheral components varies depending on the turn ratio. Select the turn ratio to optimize the overall cost, including peripheral components.

Core Size

Next, the core size is selected. Select a core size that satisfies Equation (19).

AE Effective core cross section area[cm2]
AW Core window area [cm2]
L1 Inductance value of transformer primary
IL1_max Maximum transformer primary current
IL1_rms Transformer primary RMS current
Bmax Maximum operating flux density
K 0.2Jmax×10-4(Jmax:Max current density A/cm2)
Number of turns of coil and wire diameter

Following the turn ratio and coil size of the transformer primary coil and secondary coil, the number of turns of the primary coil and secondary coil are selected. First, use Equation (20) to calculate the number of turns of the primary coil at which flux saturation will not occur in the selected core. After calculating the number of turns of the primary coil, calculate the number of turns of the secondary coil from Equation (21).

By setting the number of turns of the auxiliary coil that supplies power to the VDD pin to the value calculated from Equation (22), the VDD pin voltage can be set to the target value. Note, however, that if the number of LED series changes, the VDD pin voltage will also change.
In actuality, Spike voltages may occur and cause the VDD pin voltage to occasionally rise higher than the VDD voltage target value. Refer to section 7-5 for the countermeasure for this.

AE Effective core cross section area [cm2]
IL1_max Maximum transformer primary current
L1 Inductance value of transformer primary
Bmax Maximum operating flux density
VDD arget value of VDD pin voltage (11 to 13V)
VLED LED voltage
VF Forward voltage of rectification diode
N Turn ratio of transformer primary coil and secondary coil (=N1/N2)
N1 Number of windings of transformer primary coil
N2 Number of windings of transformer secondary coil
N3 Number of windings of transformer auxiliary coil

 Next, the selection method for the wire diameter is explained.
The wire diameter is selected based on whether the skin effect becomes apparent at the operation frequency and the current density of the maximum current that flows in the coil.
First, select the wire diameter of the primary coil and secondary coil so that the current density at the maximum current does not exceed 6A/mm2. The current in the auxiliary coil is small, so this is not a concern.

Next, to verify that the skin effect does not occur, check if the wire diameter selected above satisfies Equation (23).
If the selected wire diameter does not satisfy Equation (23), consider connecting the coils in parallel. In this case, select a wire diameter and parallel number that satisfy Equation (23) without exceeding a current density of 6A/mm2.

Evaluate possibility of building transformer based on evaluation specifications

Evaluate whether a transformer can actually be built based on the core and coil specifications selected above. Calculate the ratio of the total cross sectional area of the copper wire of all coils to the window area. This varies by application, but it can be judged that the transformer can be built if this is less than 20% of the window area in an isolated flyback circuit.

If Equation (24) is not satisfied, reconsider the transformer specifications and increase the core size, decrease the number of windings, or decrease the wire diameter.

AW Core window area [cm2]
S1 Wire cross-sectional area of primary coil of transformer (=(πd12)/2)
S2 Wire cross-sectional area of secondary coil of transformer (=(πd12)/2)
S3 Wire cross-sectional area of auxiliary coil of transformer (=(πd12)/2)
N1 Number of windings of transformer primary coil
N2 Number of windings of transformer secondary coil
N3 Number of windings of transformer auxiliary coil
p1 Parallel number of transformer primary coil
p2 Parallel number of transformer secondary coil
Transformer structure

Strengthening the coupling between coils in the transformer structure is very important for lowering leakage inductance, improving efficiency, and reducing transformer heat generation. An example of a recommended transformer structure is shown in Fig. 39.

The transformer structure of Fig. 39 is designed using TEX or space tape to satisfy the creeping distance in 220VAC/240VAC systems. Design the actual transformer structure so that it can satisfy applicable regulations and standards.

Fig. 39 Recommended schematic for transformer structure

Applicable regulations and safety standards

Verify the standards for the withstand voltage and other characteristics of the isolated transformer to be used in the isolated flyback circuit. Design the transformer that meets the applicable regulations and safety standards.

7-7. Snubber Circuit (C6,R8,D1)

The snubber circuit prevents the external power MOSFET from being destroyed by the energy stored in the transformer leakage inductance when the external power MOSFET is turned off. The snubber circuit used in this example is shown in Fig. 40, and the MOSFET drain voltage and the snubber circuit voltage when the MOSFET is off are shown in Fig. 41.

As shown in Fig. 41, the drain voltage rises steeply when the MOSFET is turned off, but the snubber circuit limits the drain voltage rise and prevents destruction of the MOSFET.

Fig.40 Snubber circuit

Fig.41 MOSFET drain voltage and snubber circuit voltage

Next, the methods for selecting the values of R8 and C6 and deciding the snubber voltage are explained.

Energy is generated by the transformer leakage inductance, and the energy is stored in the capacitor C6 when the MOSFET is turned off. The relation between the voltage applied to C6 and the leakage inductance at this time is shown in Equation (25).

ΔVC6 is the voltage drop due to discharge through R8, and when VC6 >> ΔVC6, ΔVC6 is given by Equation (26).

As an approximation, it can be assumed that the snubber voltage Vsnub is equal to VC6, so C6 and R8 can be determined from Equations (27) and (28) using Equations (25) and (26).

C6 Effective capacitance value of C6
VC6 Voltage applied to C6 immediately before MOSFET is turned off
ΔVC6 Difference in voltage applied to C6 immediately after MOSFET is turned off
Vsnub Snubber voltage (100V~150V)
Lleak Transformer leakage inductance
IL1_max Maximum transformer primary current
tOFF Off time 6.0μs(typ.)

By setting the snubber voltage Vsnub to a value from 100V to 150V and taking ΔVC6 = 5V, the resistance value for R8 and the capacitance value for C6 can be determined. If the snubber voltage Vsnub is too large, it will be necessary to increase the rated voltages of the external power MOSFET, C6, and D3, resulting in higher cost.

For the diode D3, use a fast recovery diode with a sufficiently high rated voltage and a short reverse recovery time. In actual use, parasitic inductance from the wiring and the effects of the transformer may cause deviation from the above result. For this reason, select components after verifying the snubber voltage and component heat generation in the actual equipment.

A calculation example is shown below.
When Lleak=30µH,IL1_max=0.4A,tOFF=6.0µs, and the set values are Vsnub=100V, ΔVC6=5V, R8 and C6 are

The same calculation is made in the separate calculation file. Please make use of this.

7-8. Rectifying Diode (D2)

This rectifying diode prevents reverse flow to the secondary coil of the transformer when MOSFET Q1 is in the off state and the energy stored in the transformer flows to the anode side of the LED. In an isolated flyback circuit, the maximum voltage applied to the rectifying diode D2 is given by Equation (29), and thus a product with a rated voltage higher than that must be selected.

Select a fast recovery diode or Schottky diode with a short reverse recovery time. A diode with a long reverse recovery time will adversely affect efficiency.

Vrms_max Maximum input RMS voltage
N1 Number of windings of transformer primary coil
N2 Number of windings of transformer secondary coil
VLED LED voltage

A calculation example is shown below.
When Vrms_max=240V, N2/N1=1/4, VLED=19.2V, VF=1.0V, it can be seen that the maximum voltage applied to the rectifying diode is

In this example, a product with a rated voltage of 200 V is selected.
The same calculation is made in the separate calculation file. Please make use of this.

7-9. MOSFET,Gate Resister (Q1,R7)

Power MOSFET for switching and gate resistance for switching time adjustment. By inserting a gate resistance, the MOSFET switching time can be slowed and the high-frequency EMI level reduced. However, a large gate resistance and a slower switching speed increases the switching loss of the MOSFET, decreasing efficiency. The optimum value varies depending on the MOSFET that is used, but in general a gate resistance from 5 to 50Ω should be selected.

In an isolated flyback circuit, the flyback voltage that occurs during MOSFET off time and the snubber voltage are applied to the MOSFET in addition to the AC input. (Refer to Fig. 41.) The maximum voltage VQ1 that is applied is given by Equation (30), and thus a product with a rated voltage higher than that must be selected. In addition, using a MOSFET with a small on-resistance can reduce MOSFET loss and improve efficiency.
In this example, a product with a rated voltage of 800V and a rated current of 2.5A is selected.

Vrms_max Maximum input RMS voltage
N1 Number of windings of transformer primary coil
N2 Number of windings of transformer secondary coil
VLED LED voltage
VF Forward voltage of rectification diode (D2)
Vsnub Snubber voltage (=100V~150V)

A calculation example is shown below.
When Vrms_max=240V, N1/N2=4, VLED=19.2V, VF=1.0V, Vsnub=150V, it can be seen that the maximum voltage applied to the MOSFET is

The same calculation is made in the separate calculation file. Please make use of this.

7-10. LED current adjustment (R3,R4)

Sensing resistance used to adjust the current that flows in the external power MOSFET in order to adjust the LED current. The LED current is set by adjusting the sensing resistance.

In the A type used in this example, the voltage after full wave rectification is divided by R1 and R2 and applied to the VSINE pin. By comparing the VSINE pin voltage to the ISEN pin voltage obtained by converting the current in the external power MOSFET to a voltage using R3 and R4, the current in the external power MOSFET is controlled.

The peak value of the current in the MOSFET is determined by the sensing resistances R3 and R4 according to Equation (31). However, unlike the B type, the signal input into the VSINE pin is in phase with the AC input, and thus the peak value of the current in the MOSFET changes continuously. (Refer to Fig. 34.)

Ip(t) Peak value of current in MOSFET at time t
Vrec(t) Voltage after full wave rectification at time t
R1~R4 Refer to fig.30.
α Internal constant 0.2783

In discontinuous mode in an isolated flyback circuit, the current in the MOSFET and the coil current are as shown in Fig. 35. The LED current is the average value of the current that flows in the transformer secondary coil IL2, and thus in order to set the LED current to the target value, the sensing resistance must be adjusted to satisfy Equation (32).

The value in the actual equipment may differ from the value of Equation (32), so calculate this using the separate calculation file, taking the IC internal delay and other factors into consideration.

ILED Target value of LED current
IL2(t) Current in transformer secondary coil at time t
f Commercial power frequency 50Hz/60Hz

7-11. Output Capacitor (C4)

This capacitor limits LED ripple current and ripple voltage. In this example, the A type is used to improve the power factor in an isolated flyback circuit, and thus the input current and current through the transformer secondary coil are in phase with the AC input as shown in Fig. 42. For this reason, the ripple voltage in the LED voltage fluctuates due to the cycles of two frequencies, the commercial frequency and the switching frequency.

However, in this example the fluctuation in LED voltage due to the cycle of the commercial frequency is far larger than the fluctuation due to the cycle of the switching frequency, and thus the component due to the switching frequency cycle can be disregarded when calculating the output capacitance.

The value of the output capacitance is determined by the ripple current ratio of the LED current. Here we decide the capacitance value taking a ripple current ratio of 0.8 as the target value. If the ripple current ratio is to be kept under 0.8 when ILED = 350mA (ripple current 350mA ×0.8 = 280mA), we first calculate the allowed Vripple from the IV characteristic of the LED to be used. Here, 0.35V×6=2.1V from Fig. 43.

Fig.42 Transformer current and LED current

Fig.43 LED IV characteristic

The ripple voltage can be expressed by Equation (33) as a relation of the transformer secondary coil current, LED current, and output capacitance. Use this to select a capacitance value that gives a ripple voltage of 2.1V.
If an electrolytic capacitor will be used for the output capacitance, select a product with sufficient allowance for ripple current.

Vripple Allowed ripple voltage in LED voltage
IL 2(t) Value of current in transformer secondary coil at time t
ILED(t) LED current value at time t
C Effective capacitance value of output capacitance C4
t1、t2 Time t1 to t2 over which IL 2(t) and ILED(t) equalize

The actual calculation is complex, so please check in the calculation file.
In addition, the actual ESR effects of the capacitor and IV characteristics of the LED are non-linear, and thus the value may vary in the actual equipment. Test in the actual equipment before selecting the capacitance value.

Torex Products in this article

Close