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I am trying to build a isolated forward converter with voltage mode feedback. For some reason, I am unable to achieve required crossover frequency. To simplify everything, I have change the circuit to a buck converter. To simulate it, I am using LTspice with FRA analysis and LT1578 as a controller.

Designed parameters:

• Vin = 8 V
• Vout = 3.3 V
• Ioutmax = 1 A
• fsw = 200 kHz
• Lout = 100 µH
• Cout = 100 µF

As I am using ceramic capacitors, I assume ESR of output cap to be zero. And therefore I need a Type 3 compensator.

As the filter has a peak at 1.6 kHz, I put a double zero at half - 800 Hz. Double pole at half of the switching frequency - 100 kHz. Crossover frequency to the 10% of the switching frequency - 10 kHz.

Then I use these equations to calculate all components:

The problem is that when I simulate the circuit, no matter what I set as the gain of the EA, I always get the crossover frequency similar to the frequency of the double zero.

For gain = 10, fz = 800 Hz, fco = 10 kHz, fp = 100 kHz:

For gain = 100, fz = 800 Hz, fco = 10 kHz, fp = 100 kHz:

For gain = 10, fz = 3000 Hz, fco = 10 kHz, fp = 100 kHz:

For gain = 100, fz = 3000 Hz, fco = 10 kHz, fp = 100 kHz:

The simulated circuit for gain = 10, fz = 800 Hz, fco = 10 kHz, fp = 100 kHz:

Link for download of the circuit: LTspice circuit

Can you please help me out where I made a mistake?

EDIT: I have changed the circuit to the one I have actually build. The results of the simulations are exactly the same.

I have calculated the gain of the power stage to be at low freq: Apwr = Vin / Vramp = 8/0.9 = 19 dB.

I also verified the gain by setting the gain of error amplifier to 1 and appling low frequency sin source to the amplifier and measured the output:

The simulated gain = 2.22/0.2 = 20 dB

I used the gain and LC filter parameters to simulate the circuit in Simplis: I had to lower the gain at crossover parameter a little bit to achieve desired frequency. The results:

Simplis circuit

Then I used the calculated parameters to the LTspice circuit. Output voltage of the LC filter:

The problem is that with the same parameters the circuit is not stable and not able to achive the stable output voltage.

I have measured the circuit in the lab, the output voltage is stable, but I measured the crossover frequency to be about 300 Hz.

So what am I doing wrong? Thank you

I am trying to build a isolated forward converter with voltage mode feedback. For some reason, I am unable to achieve required crossover frequency. To simplify everything, I have change the circuit to a buck converter. To simulate it, I am using LTspice with FRA analysis and LT1247 as a controller.

I am using the part in voltage mode as described in an article from TI (for similar part):

Designed parameters:

• Vin = 8 V
• Vout = 3.3 V
• Ioutmax = 1 A
• fsw = 200 kHz
• Lout = 100 µH
• Cout = 100 µF

As I am using ceramic capacitors, I assume ESR of output cap to be zero. And therefore I need a Type 3 compensator.

As the filter has a peak at 1.6 kHz, I put a double zero at half - 800 Hz. Double pole at half of the switching frequency - 100 kHz. Crossover frequency to the 10% of the switching frequency - 10 kHz.

Then I use these equations to calculate all components:

The problem is that when I simulate the circuit, no matter what I set as the gain of the EA, I always get the crossover frequency similar to the frequency of the double zero.

For gain = 10, fz = 800 Hz, fco = 10 kHz, fp = 100 kHz:

For gain = 100, fz = 800 Hz, fco = 10 kHz, fp = 100 kHz:

For gain = 10, fz = 3000 Hz, fco = 10 kHz, fp = 100 kHz:

For gain = 100, fz = 3000 Hz, fco = 10 kHz, fp = 100 kHz:

The simulated circuit for gain = 10, fz = 800 Hz, fco = 10 kHz, fp = 100 kHz:

Link for download of the circuit: LTspice circuit

Can you please help me out where I made a mistake?

EDIT: I have changed the circuit to the one I have actually build. The results of the simulations are exactly the same.

I have calculated the gain of the power stage to be at low freq: Apwr = Vin / Vramp = 8/0.9 = 19 dB.

I also verified the gain by setting the gain of error amplifier to 1 and appling low frequency sin source to the amplifier and measured the output:

The simulated gain = 2.22/0.2 = 20 dB

I used the gain and LC filter parameters to simulate the circuit in Simplis: I had to lower the gain at crossover parameter a little bit to achieve desired frequency. The results:

Simplis circuit

Then I used the calculated parameters to the LTspice circuit. Output voltage of the LC filter:

The problem is that with the same parameters the circuit is not stable and not able to achive the stable output voltage.

I have measured the circuit in the lab, the output voltage is stable, but I measured the crossover frequency to be about 300 Hz.

So what am I doing wrong? Thank you

I am trying to build a isolated forward converter with voltage mode feedback. For some reason, I am unable to achieve required crossover frequency. To simplify everything, I have change the circuit to a buck converter. To simulate it, I am using LTSpice with FRA analysis and LT1578 as a controller.

Designed parameters: Vin = 8V, Vout = 3,3V, Ioutmax = 1A, fsw = 200kHz, Lout = 100uH, Cout = 100uF.

As I am using ceramic capacitors, I assume ESR of output cap to be zero. And therefore I need a Type 3 compensator.

As the filter has a peak at 1,6 kHz, I put a double zero at half - 800 Hz. Double pole at half of the switching frequency - 100 kHz. Crossover frequency to the 10% of the switching frequency - 10kHz.

Then I use these equations to calculate all components:

The problem is that when I simulate the circuit, no matter what I set as the gain of the EA, I always get the crossover frequency similar to the frequency of the double zero.

For gain = 10, fz = 800 Hz, fco = 10 kHz, fp = 100 kHz:

For gain = 100, fz = 800 Hz, fco = 10 kHz, fp = 100 kHz:

For gain = 10, fz = 3000 Hz, fco = 10 kHz, fp = 100 kHz:

For gain = 100, fz = 3000 Hz, fco = 10 kHz, fp = 100 kHz:

The simulated circuit for gain = 10, fz = 800 Hz, fco = 10 kHz, fp = 100 kHz:

Link for download of the circuit: LTSpice circuit

Can you please help me out where I made a mistake?

EDIT: I have changed the circuit to the one I have actually build. The results of the simulations are exactly the same.

I have calculated the gain of the power stage to be at low freq: Apwr = Vin / Vramp = 8/0.9 = 19 dB.

I also verified the gain by setting the gain of error amplifier to 1 and appling low frequency sin source to the amplifier and measured the output:

The simulated gain = 2.22/0.2 = 20 dB

I used the gain and LC filter parameters to simulate the circuit in Simplis: I had to lower the gain at crossover parameter a little bit to achieve desired frequency. The results:

Simplis circuit

Then I used the calculated parameters to the LTSpice circuit. Output voltage of the LC filter:

The problem is that with the same parameters the circuit is not stable and not able to achive the stable output voltage.

I have measured the circuit in the lab, the output voltage is stable, but I measured the crossover frequency to be about 300 Hz.

So what am I doing wrong? Thank you

I am trying to build a isolated forward converter with voltage mode feedback. For some reason, I am unable to achieve required crossover frequency. To simplify everything, I have change the circuit to a buck converter. To simulate it, I am using LTspice with FRA analysis and LT1578 as a controller.

Designed parameters:

• Vin = 8 V
• Vout = 3.3 V
• Ioutmax = 1 A
• fsw = 200 kHz
• Lout = 100 µH
• Cout = 100 µF

As I am using ceramic capacitors, I assume ESR of output cap to be zero. And therefore I need a Type 3 compensator.

As the filter has a peak at 1.6 kHz, I put a double zero at half - 800 Hz. Double pole at half of the switching frequency - 100 kHz. Crossover frequency to the 10% of the switching frequency - 10 kHz.

Then I use these equations to calculate all components:

The problem is that when I simulate the circuit, no matter what I set as the gain of the EA, I always get the crossover frequency similar to the frequency of the double zero.

For gain = 10, fz = 800 Hz, fco = 10 kHz, fp = 100 kHz:

For gain = 100, fz = 800 Hz, fco = 10 kHz, fp = 100 kHz:

For gain = 10, fz = 3000 Hz, fco = 10 kHz, fp = 100 kHz:

For gain = 100, fz = 3000 Hz, fco = 10 kHz, fp = 100 kHz:

The simulated circuit for gain = 10, fz = 800 Hz, fco = 10 kHz, fp = 100 kHz:

Link for download of the circuit: LTspice circuit

Can you please help me out where I made a mistake?

EDIT: I have changed the circuit to the one I have actually build. The results of the simulations are exactly the same.

I have calculated the gain of the power stage to be at low freq: Apwr = Vin / Vramp = 8/0.9 = 19 dB.

I also verified the gain by setting the gain of error amplifier to 1 and appling low frequency sin source to the amplifier and measured the output:

The simulated gain = 2.22/0.2 = 20 dB

I used the gain and LC filter parameters to simulate the circuit in Simplis: I had to lower the gain at crossover parameter a little bit to achieve desired frequency. The results:

Simplis circuit

Then I used the calculated parameters to the LTspice circuit. Output voltage of the LC filter:

The problem is that with the same parameters the circuit is not stable and not able to achive the stable output voltage.

I have measured the circuit in the lab, the output voltage is stable, but I measured the crossover frequency to be about 300 Hz.

So what am I doing wrong? Thank you

I am trying to build a isolated forward converter with voltage mode feedback. For some reason, I am unable to achieve required crossover frequency. To simplify everything, I have change the circuit to a buck converter. To simulate it, I am using LTSpice with FRA analysis and LT1578 as a controller.

Designed parameters: Vin = 8V, Vout = 3,3V, Ioutmax = 1A, fsw = 200kHz, Lout = 100uH, Cout = 100uF.

As I am using ceramic capacitors, I assume ESR of output cap to be zero. And therefore I need a Type 3 compensator.

As the filter has a peak at 1,6 kHz, I put a double zero at half - 800 Hz. Double pole at half of the switching frequency - 100 kHz. Crossover frequency to the 10% of the switching frequency - 20kHz.

Then I use these equations to calculate all components:

The problem is that when I simulate the circuit, no matter what I set as the gain of the EA, I always get the crossover frequency similar to the frequency of the double zero.

For gain = 10, fz = 800 Hz, fco = 20 kHz, fp = 100 kHz:

For gain = 100, fz = 800 Hz, fco = 20 kHz, fp = 100 kHz:

For gain = 10, fz = 3000 Hz, fco = 20 kHz, fp = 100 kHz:

For gain = 100, fz = 3000 Hz, fco = 20 kHz, fp = 100 kHz:

The simulated circuit for gain = 10, fz = 800 Hz, fco = 20 kHz, fp = 100 kHz:

Link for download of the circuit: LTSpice simulation

Can you please help me out where I made a mistake?

I am trying to build a isolated forward converter with voltage mode feedback. For some reason, I am unable to achieve required crossover frequency. To simplify everything, I have change the circuit to a buck converter. To simulate it, I am using LTSpice with FRA analysis and LT1578 as a controller.

Designed parameters: Vin = 8V, Vout = 3,3V, Ioutmax = 1A, fsw = 200kHz, Lout = 100uH, Cout = 100uF.

As I am using ceramic capacitors, I assume ESR of output cap to be zero. And therefore I need a Type 3 compensator.

As the filter has a peak at 1,6 kHz, I put a double zero at half - 800 Hz. Double pole at half of the switching frequency - 100 kHz. Crossover frequency to the 10% of the switching frequency - 10kHz.

Then I use these equations to calculate all components:

The problem is that when I simulate the circuit, no matter what I set as the gain of the EA, I always get the crossover frequency similar to the frequency of the double zero.

For gain = 10, fz = 800 Hz, fco = 10 kHz, fp = 100 kHz:

For gain = 100, fz = 800 Hz, fco = 10 kHz, fp = 100 kHz:

For gain = 10, fz = 3000 Hz, fco = 10 kHz, fp = 100 kHz:

For gain = 100, fz = 3000 Hz, fco = 10 kHz, fp = 100 kHz:

The simulated circuit for gain = 10, fz = 800 Hz, fco = 10 kHz, fp = 100 kHz:

Link for download of the circuit: LTSpice circuit

Can you please help me out where I made a mistake?

EDIT: I have changed the circuit to the one I have actually build. The results of the simulations are exactly the same.

I have calculated the gain of the power stage to be at low freq: Apwr = Vin / Vramp = 8/0.9 = 19 dB.

I also verified the gain by setting the gain of error amplifier to 1 and appling low frequency sin source to the amplifier and measured the output:

The simulated gain = 2.22/0.2 = 20 dB

I used the gain and LC filter parameters to simulate the circuit in Simplis: I had to lower the gain at crossover parameter a little bit to achieve desired frequency. The results:

Simplis circuit

Then I used the calculated parameters to the LTSpice circuit. Output voltage of the LC filter:

The problem is that with the same parameters the circuit is not stable and not able to achive the stable output voltage.

I have measured the circuit in the lab, the output voltage is stable, but I measured the crossover frequency to be about 300 Hz.

So what am I doing wrong? Thank you