9
\$\begingroup\$

This is my first time designing a full bridge driver. I am experiencing problems with ringing on the output. I have made a pcb for it. This is a picture of the top side of the board. PCB Front

Backside PCB Back

Input to L6498 Drivers, 250ns dead time enter image description here

Unloaded output voltage of the full bridge enter image description here

Output with unloaded transformer attached CH1: Transformer Voltage CH2: Transformer current enter image description here

Full setup enter image description here

The problem I have is with the oscillation at the top of the output waveform when a load is attached. Applying a load to the transformer only makes ringing worse. I have tested the gates of all the mosfets and the waveforms are very clean with no spikes even when the transformer is loaded. The only problem is with the bridge output waveform. The board has a 1uf film capacitor in the center of the board. I have tried adding a 2200uf capacitor right at the main voltage rail next to the mosfet as shown in the image below. I also have a current transformer to measure the capacitor current.

enter image description here The output waveform improves with transformer still connected when electrolytic cap is added. CH1 : Full bridge output voltage CH2: Electrolytic capacitor current. enter image description here The problem with this is: the electrolytic cap gets warm under very light loading of the full bridge. At high loads the current through the capacitor was about 30 amps at the peak. The capacitor was very hot. If adding more capacitance to the supply rail would improve the ringing, what kind of capacitor should I use? Would a larger film capacitor help the ringing? Is the ringing a layout problem? If so, should the pcb power traces be shorter?

\$\endgroup\$
7
  • 2
    \$\begingroup\$ Welcome to EE.SE. For intense current pulses only Polypropylene capacitors will survive and stay cool, but finding one over 10uF would be a challenge and expensive. Try to keep the Golden triangle of current source (Large cap rated for switch mode supplies), current switch (your MOSFET's), and current load (resistor or motor) as close as possible. You should have some Polypropylene capacitors on hand just to try them. They are just about indestructible. \$\endgroup\$
    – user105652
    Commented Oct 28, 2018 at 6:02
  • 1
    \$\begingroup\$ So, what you perceive to be a problem is some ringing yes? Why is this perceived as a problem? Where is your schematic? Where is your simulation result? \$\endgroup\$
    – Andy aka
    Commented Oct 28, 2018 at 9:55
  • \$\begingroup\$ @ Sparky The concept of Golden Triangle is new phrase to me, yet is very descriptive: (1) Current Source, (2) Current Switch, (3) Current Load, should be a very tight triangle. I'd add that the 3 pieces should be arranged for immediately-adjacent physical placement, so the magnetic fields will LARGELY CANCEL and the energy stored in the (now smaller loop area) loop is much less. \$\endgroup\$
    – analogsystemsrf
    Commented Oct 28, 2018 at 11:31
  • 2
    \$\begingroup\$ Terrible layout! Why is everything so far apart? Please add your schematic. \$\endgroup\$
    – winny
    Commented Oct 28, 2018 at 11:53
  • \$\begingroup\$ Measure the frequency of the ringing and try replacing your gate resistors with ferrite beads of the same footprint that peak at that frequency. Or just increase your gate resistors. I agree the layout is bad though. You have no ground plane underneath your gate drive traces so the return current impedances are very high. The current loop your gate drive currents travel are very large which would also contribute to these problems. You should have designed your MOSFETs to go right on the board, not through screw terminal connectors since stuff like that matters here. Too many parasitics. \$\endgroup\$
    – DKNguyen
    Commented Apr 14, 2019 at 15:57

4 Answers 4

Reset to default
1
\$\begingroup\$

You must use, Fast turn-on / Slow turn-off to drive the gates... And reduce your loop of driving gates..

\$\endgroup\$
1
  • 4
    \$\begingroup\$ That is a bit short for an answer. If you could add some details, this might become a good answer. \$\endgroup\$
    – JRE
    Commented May 10, 2019 at 20:39
0
\$\begingroup\$

Since you have not added a schematic and based on the information you provided i can advice only from my own experience and understanding:

1- The electrolytic capacitor is more important here , since the ripple current will go to the higher capacitance . Use film only for high frequency noise , capacitance range of 100nF or 10nF.

2- The electrolytic capacitor traces should be as short as possible those 2 wires are the problem. Solder directly to pcb close to power mosfets (not like those 2 caps at the blue pcb outside).

3- Add more electrolytic capacitors since it is getting hot which means you the ripple current is higher than what it is rated for.

4-I cannot see where power is entering your board, those traces should be wide as possible to reduce inductance.

5- Try suppling the board with batteries if you are using bench power supply currently

Start with these general notes and let us know if the problem persists.

Hope this helps

\$\endgroup\$
0
\$\begingroup\$

You can try adding a low pass filter at the output with strong capacitor (micro farad) If your scope can perform FFT, try it on your pulse signal to help you better understand your circuit frequency interfering and design the LPF properly. It can be a simple RC circuit with the right values...

\$\endgroup\$
0
\$\begingroup\$

You would probably have better results with R/C snubber networks, try experimenting with values of capacitor around 0.05 to 0.1 μF in series with resistors around 10 to 47 Ω.

The optimum values for the snubber will depend on the load.

Keep leads as short as possible.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.