High voltage spikes on the rising edges in a free wheeling diode

Hi

i am using msr860 as a free wheeling diode across the h- bridge mosfets for both the high and the low but after a short while of operation it fails and goes cocked and then shorted "1 minute ".

it was noted that the -ve portions due to the back emf of the voltage signal had been removed after adding the freewheeling diodes but still there is a high voltage spikes on the riesing edges of the switching signal .
where the h- bridge is 24VDC 15 amp , and 20khz .

i don't know what may cause although it has a sutable ratings for may load .

so , what may cause this problem ?

and how to remove this rising edge spikes ?

salam
hindi

Re: free wheeling diode

HINDI said:

Hi

i am using msr860 as a free wheeling diode across the h- bridge mosfets for both the high and the low but after a short while of operation it fails and goes cocked and then shorted "1 minute ".

it was noted that the -ve portions due to the back emf of the voltage signal had been removed after adding the freewheeling diodes but still there is a high voltage spikes on the riesing edges of the switching signal .
where the h- bridge is 24VDC 15 amp , and 20khz .

i don't know what may cause although it has a sutable ratings for may load .

so , what may cause this problem ?

and how to remove this rising edge spikes ?

salam
hindi

Click to expand...

You have parasitic inductace on wrong place in high frequency domain, possible in conductor between components... - and component itself...

if you have ringing and spike in ns-time domain - you handle in really world frequency in many MHz on your circurit - and need thinking RF...

in many time not easy to point out exact place of inductance
ie distributed on long cords in wide area loop and depend of
layout and cords orientation.

You need thinking with HF and RF eyes and make fast shifting current
on a small short and closed loop as possibly.

---

most common, scanty, methode to fix this problem (after design...) is a slow down fast rising edges with capacitance and RC-network on rigth place - and also freewheel diodes.

this soloution called 'snubber network' and possible to find lot of information her in EDAboard if seaching with word 'snubber', and also on google...

you need understud 'why' - to make best possible solution on your circurit...

Re: free wheeling diode

a snubber solution is what i was thinking, but since that has already been posted i offer perhaps a simplier solution.

slow down your turn ON time of your switching element. most common way is to increase the series resistor going to your gate. if you want to control the turn ON/OFF times seperately then you will need minimum 2 resistors and 1 diode per gate.

the trade off is you will burn more power in your switching losses. probably you will burn same amount if you use a snubber, the difference is that a snubber will localize the power dissipation in the snubber (i.e. NOT your switching device!).

Mr.Cool

Re: free wheeling diode

The diode may be burning because of temperature try to put a heatsink or use a ultrafast rectifier.