How to solve the Hard Switching iin the converter circuit?

[madhu.b]

Hello,

I am working on DC-DC Converter circuit with specification of
Operating Frequency - 3.45Mhz,
Vin - 30V,
Iin -Approximatly 0.5A,
Load Resistor - 15 ohm,
Duty Cycle - 50%.
In the simulations I didn't get that much hard switching but while coming to the hardware(PCB) in which it is more. So that the hard switching will effect on the MOSFET and it will get hot and will burned the MOSFET. So, How can I get back from the hard switching into the soft switching (ZVS).

 

[mtwieg]

A few approaches are viable, such as using a resonant topology (LLC or phase shift bridge converter) or using better FET technology which mitigates the dissipation from hard switching (like GaN FETs).

 

[FvM]

What's the circuit?

 

[madhu.b]

I am using good MOSFET which is the IPL60R385CP and using LC resonant topology.
Here, I have attached the part of the circuit.

View attachment untitled.bmp

 

[mtwieg]

Oh, this circuit again...

If you built it properly, then it should be switching with ZVS, even if the FET is not optimal. What makes you think there are excessive switching losses? Do you have any scope waveforms to show?

 

[FvM]

If you built it properly, then it should be switching with ZVS

Should it? There are several prerequisites, I think. Component dimensioning, drive waveform...

 

[mtwieg]

Right, proper pulse timings goes with my criterion for "built properly."

I believe his circuit fits into the SRPS topology, so it should be driven with a near-constant off time, and variable on time, so duty cycle will vary with line/load conditions. If the FET off time is too short, you won't get ZVS, and if it's too long you'll get conduction of the FET body diode, which will also increase losses.

 

[madhu.b]

I have another problem is that, when I am working on the Vin-20V, Load Resistance-15ohm. Then the Vin come down to 15V or 16V and at the input current Iin-0.6A. Then the MOSFET getting too much hot.

What is the reason behind to get the MOSFET hot?
Why the voltage drops to 4V to 5V?
If the voltage drops the current goes high?
Do you think the problem with MOSFET or Chock Inductor? I am using IPL60R385CP MOSFET and Chock Inductor - CTX50-3A-R.
Please tell me that how can I over come these problem?

Here, I am attaching the waveform for Soft-Switching.

https://obrazki.elektroda.pl/3986004800_1361790341.gif

 

[FvM]

Doesn't look much like soft switching, neither ZCS nor ZVS.

 

[mtwieg]

Yeah, you're way off from ZVS. Might want to double check that all your component values are at least close to what your simulation uses. But from that plot it looks like your off time is at least a factor of three too short.

 

[madhu.b]

Hello FvM/mtwieg,

Except the MOSFET, I have used the same component values which I have used in the simulations. Do you think that due to the MOSFET the hard switching problem may occured? If you so, what I suppose to do to over come this problem. By the way, how to calculate the parasitics in the circuit?

 

[FvM]

You should also show your simulated waveforms.

 

[madhu.b]

I have drive the circuit with specifications of operating frequency-3.4MHz and duty cycle-50%.
The number 1 waveform indicate that the drain signal and number 2 waveform indicate that the gate signal. I can't able to capture the drain signal with in the oscilloscope screen because of the high voltage at the drain terminal.

https://obrazki.elektroda.pl/1526893500_1361878556.jpg

 

[mtwieg]

It's possible that the FET has different junction capacitances than the simulation, which would throw off the waveforms. Or parasitic capacitance in your transformer.

You should just try adjusting the frequency and duty cycle until you get proper ZVS operation, then show us what it looks like.

 

[madhu.b]

Yes, I tried with difference frequencies and duty cycle (If change the duty also it behave's same manner so I haven't change any duty).. I get the ZVS at 400KHz. But, In simulations I get the 3.4MHz.. So, It has huge difference..

Here, I have attached the waveforms for from 3.5MHz to 400KHz with 50% duty cycle. Here You Go.........

3.5MHz - https://obrazki.elektroda.pl/5504466300_1362055181.jpg
3.4MHz - https://obrazki.elektroda.pl/6221898200_1362055240.jpg
3.3MHz - https://obrazki.elektroda.pl/6306185100_1362055278.jpg
3.2MHz - https://obrazki.elektroda.pl/5976291500_1362055326.jpg
3.1MHz - https://obrazki.elektroda.pl/9117727100_1362055359.jpg
3MHz - https://obrazki.elektroda.pl/5650887700_1362055403.jpg
2.9MHz - https://obrazki.elektroda.pl/6569653500_1362055448.jpg
2.8MHz - https://obrazki.elektroda.pl/9325755700_1362055485.jpg
2.7MHz - https://obrazki.elektroda.pl/9520084800_1362055511.jpg
2.6MHz - https://obrazki.elektroda.pl/1520558400_1362055543.jpg
2.5MHz - https://obrazki.elektroda.pl/3964179200_1362055570.jpg
2.4MHz - https://obrazki.elektroda.pl/6103652700_1362055602.jpg
2.3MHz - https://obrazki.elektroda.pl/5436512800_1362055627.jpg
2.2MHz - https://obrazki.elektroda.pl/8469945600_1362055650.jpg
2.1MHz - https://obrazki.elektroda.pl/4081506800_1362055676.jpg
2MHz - https://obrazki.elektroda.pl/9210427800_1362055733.jpg
1.9MHz - https://obrazki.elektroda.pl/5510398700_1362055761.jpg
1.8MHz - https://obrazki.elektroda.pl/2960551000_1362055788.jpg
1.7MHz - https://obrazki.elektroda.pl/4292734200_1362055811.jpg
1.6MHz - https://obrazki.elektroda.pl/3734511500_1362055841.jpg
1.5MHz - https://obrazki.elektroda.pl/1353507300_1362055867.jpg
1.4MHz - https://obrazki.elektroda.pl/1234647300_1362055889.jpg
1.3MHz - https://obrazki.elektroda.pl/5835209000_1362055910.jpg
1.2MHz - https://obrazki.elektroda.pl/9037963100_1362055938.jpg
1.1MHz - https://obrazki.elektroda.pl/5994894500_1362055967.jpg
1MHz - https://obrazki.elektroda.pl/7439203300_1362056020.jpg
900KHz - https://obrazki.elektroda.pl/3156290200_1362056062.jpg
800KHz - https://obrazki.elektroda.pl/2900370400_1362056089.jpg
700KHz - https://obrazki.elektroda.pl/6774593900_1362056114.jpg
600KHz - https://obrazki.elektroda.pl/7314173500_1362056142.jpg
500KHz - https://obrazki.elektroda.pl/4082602200_1362056172.jpg
400KHz - https://obrazki.elektroda.pl/1943809200_1362056203.jpg


So, Please give me the idea how to get the ZVS at 3.4MHz around....