How to Determine Capacitor Value
How to Determine Capacitor Value
(OP)
I am trying to determine the capacitor value needed to suppress a voltage spike that occurs when a MOSFET turns on.
I have a pair of MOSFETs (QA and QB) in an H-Bridge configuration which are used to switch a 12V battery across a load. I also power the logic to control the H-Bridge from this battery. Power to this logic is switched on and off with a bipolar transistor (Q1) driven remotely through an opto-coupler. I have uploaded a circuit diagram.
When QA turns on and QB turns off, there is a voltage spike that destroys Q1. If I place a big enough capacitor (C) across the battery it works fine. I am trying to determine how big I need to make C in order to make this work. It seems to be dependent on factors in the implementation of the circuit. I have built several copies of this circuit and the value of C that allows it to work is different for each. As expected, the largest value works for all three, but how do I know this will work for the next one? It also seems to be dependent on the battery as well. I assume this is because the voltage of the battery is different depending on its state of charge and the internal impedance of the battery probably has something to do with it as well.
I have a pair of MOSFETs (QA and QB) in an H-Bridge configuration which are used to switch a 12V battery across a load. I also power the logic to control the H-Bridge from this battery. Power to this logic is switched on and off with a bipolar transistor (Q1) driven remotely through an opto-coupler. I have uploaded a circuit diagram.
When QA turns on and QB turns off, there is a voltage spike that destroys Q1. If I place a big enough capacitor (C) across the battery it works fine. I am trying to determine how big I need to make C in order to make this work. It seems to be dependent on factors in the implementation of the circuit. I have built several copies of this circuit and the value of C that allows it to work is different for each. As expected, the largest value works for all three, but how do I know this will work for the next one? It also seems to be dependent on the battery as well. I assume this is because the voltage of the battery is different depending on its state of charge and the internal impedance of the battery probably has something to do with it as well.





RE: How to Determine Capacitor Value
Is this load regenerative?
What IS the load?
Keith Cress
kcress - http://www.flaminsystems.com
RE: How to Determine Capacitor Value
RE: How to Determine Capacitor Value
Dan - Owner

http://www.Hi-TecDesigns.com
RE: How to Determine Capacitor Value
So far I haven't hooked up a load. I figured I would make sure all the logic work first, then address the issues associated with the actual load.
Thanks again.
Ed
RE: How to Determine Capacitor Value
Most big caps are electrolythic, good for sinking high amounts of charge, though rather slow for too fast spikes.
Try adding some ceramic/tantalum cap in parallel, whose shorter response time will sink some of the rising edge of spikes, leaving the slower big cap to absorb the rest of the spike energy.
Good Luck!
RE: How to Determine Capacitor Value
That is an interesting comment. Makes me think both fets are on at the same time for a short period greatly lowering battery voltage. The cap on the input of the three terminal regulator then supplies a REVERSE voltage on the transistor destroying it. An isolating diode between the battery and the transistor switch circuit would solve that. Of course, tis also means you have a design error in the logic circuit that allows both fets to be on at the same time. Adding the diode will allow you to diagnose this problem with a scope without destroying additional components.
RE: How to Determine Capacitor Value
Thanks.
Ed
RE: How to Determine Capacitor Value
You need to minimise the loop area if possible to give the best results.
Best regards,
Mark Empson
L M Photonics Ltd
RE: How to Determine Capacitor Value