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i want to build induction furnace

i parallel 16 power mosfet(irfp460) 8 fet for high side and 8 fet for low side for half bridge

i use ir2104s and bd139 and bd140 totem pole for drive mosfet.

like this

enter image description here

According to irfp460 datasheet, Total Gate Charge=210 nc and td(on)=18 and tr=59

td(on)+tr=77ns then 210/77=2.7 amp. V=IR then R=6.6 and i use 6.8 ohm resistor for each gate and use 10 ohm for bd139 and bd140 bias.

i see the output of totem pole with oscop and its nice and clean square wave without any noise or ringing. like this

enter image description here

but when i connect output of totem pole to high side mosfet the output is ringing and have Too much noise with 12 volt switching power supply without any load for test.

like this

enter image description here

freq is 20khz

why this is happen?

how to reduce or Eliminate this ringing and noise?

thank you

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  • \$\begingroup\$ I think you should post a full circuit diagram (ok don't draw all the mosfets, but at least two on each side). What I suspect happens in your circuit is that the gate capacitance of all those parallel mosfets adds up and together with the parasitic inductance of the wires/traces to the mosfets' gates forms an LC tank which causes the ringing. To test if this is the case, do you get cleaner output if you connect fewer mosfets to your drivers? If this is the cause, a solution would be Zener clamps at the gates. \$\endgroup\$
    – got trolled too much this week
    Commented Jan 13, 2015 at 5:49
  • \$\begingroup\$ You may also try small bypass capacitors to remove the high frequency ringing. Also, put a load on the output. \$\endgroup\$
    – Guill
    Commented Jan 16, 2015 at 3:51
  • \$\begingroup\$ Downvoted because of the confusing text + schematic. It's hard to see whether there's a mistake in the schematic when it's only described by text. The schematic provided shows only the H-bridge driver apparently. The question's community value is low. \$\endgroup\$
    – try-catch-finally
    Commented Feb 11, 2017 at 10:25

2 Answers 2

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During switching transients, each MOSFET (as noted by asndre) presents a large and changing capacitive load. This is best understood by viewing the relationship between Vgs, Vds and Id. Relationship between MOSFET switching parameters

As you can see, drain to source voltage decreases during a time when the gate is charging (the channel is being pulled in). During this time, the effective capacitive load on your driver is changing. In addition, each MOSFET will have slightly different parameters and each one will very probably be providing a different amount of current to the load.

This charge (during switch-on) and discharge (during switch-off) is the underlying culprit of the ringing you see on the drive circuit.

In general, a gate resistor should be used for each MOSFET from your driver, although some manufacturers (NXP in particular) recommend a ferrite bead for isolation of the gate drives. This isolation helps control the ringing you see (it is almost impossible to eliminate it), so a zener and diode circuit is sometimes recommended. A small capacitor from gate to ground may also be beneficial.

The exact value(s) are layout dependent, so not only a schematic but also a picture of your layout would be useful.

Note that turn on and turn off time has a dependency on the drive capability of the gate driver as its ability to source and sink current determines how quickly the device will go through the transition region.

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  • \$\begingroup\$ +1. What size capacitor, as a rule of thumb? Electrolytic? Is 100ohm resistor common? \$\endgroup\$
    – user2497
    Commented Jan 8, 2018 at 20:42
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    \$\begingroup\$ Typically in the low nF range, available in non-polarised. Most gate resistors are in the range of 2 (or so) ohms to perhaps 20 ohms. It is very design specific. \$\endgroup\$
    – Peter Smith
    Commented Jan 9, 2018 at 12:04
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why this is happen?

Because each MOSFET has a parasitic capacitance on its gate and when you parallel MOSFETs you increase the total capacitance proportionally.

how to reduce or eliminate this ringing and noise?

You need to boost your driver (the signal that drives the gates in an MOSFET arm), i.e. decrease its output resistance. Also you need to drive each MOSFET arm separately to prevent dead time effects (when both two arms are open).

P.S. Why are you drawing your MOSFETs as bi-polar junction transistors? Be careful with it.

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