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Driver Circuit for a Reciprocating Water Pump in a Medical Device

jw0752

Hello Friends,

I am working on servicing a circuit board that is used in a Medical Dental sterilizer. The circuit boards used in this model of sterilizer seem to have a chronic problem with a circuit that drives a reciprocating water pump. Here is a schematic of the circuit that fails:

image

My question is what is the advantage to using this circuit configuaration to drive the pump? The gate of the N CH MOSFET turns on and pulls one leg of the load to the negative rail and also turns on the P CH MOSFET about 20 microseconds later pulling the other leg of the load up to the positive rail. While this seems to work quite well it seems to me to be an added complication to be switching both legs of the load. I am hoping for some insight into what the advantage might be in using this configuaration. The Rds (on) of the N CH is about 60 milli Ohm and the Rds of the P CH is about 13 milli Ohm. While I am not certain this is probably a Pulsed circuit as I suspect that the pump needs a Pulsed drive to properly function. My hope is that in better understanding the advantages of this circuit design I might be able to modify it to lower the present failure rate. Failure is usually a shorted P CH Mosfet, A Heat Destroyed N CH MOSFET and a Shorted D26 Diode.

Any insights you may have will be appreciated.

John

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  • 0

    Hello John,

    I am not very well versed in high power MOSFET switching, but a quick search of know switching circuits does not find any similar circuits. Most circuits tend singular MOSFETs in either a high side or low side configurations.  The only High and Low side configurations that I can find are half-bridge designs, were bi-directional currents are needed.

    I hope and trust all is well with you.  It is great to see on-line.

  • 0 in reply to genebren

    Hi Gene,

    Thanks for searching the switching circuits for me. I have never seen this configuration before and when ones skill level is as low as mine the first thought is that the designer had some omnicient insight that is beyond me. I find Michael's suggestion that this is a design that has evolved from its original intention fascinating. When circuits are on high quality circuit boards in very expensive equipment the thought that they may be jury rigged from previous designs didn't occur to me.

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  • 0 in reply to genebren

    Hi Gene,

    Thanks for searching the switching circuits for me. I have never seen this configuration before and when ones skill level is as low as mine the first thought is that the designer had some omnicient insight that is beyond me. I find Michael's suggestion that this is a design that has evolved from its original intention fascinating. When circuits are on high quality circuit boards in very expensive equipment the thought that they may be jury rigged from previous designs didn't occur to me.

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  • 0 in reply to jw0752

    Hello John,

    I would think that the designer was familiar with this converter topology

    https://www.vishay.com/docs/91616/twoswitch.pdf

    In the case of the forward converter with a transformer, it guarantees the magnetic reset of the core (with PWM less than 50%), stopping it stepping to saturation, and has a small benefit in recycling much of the magnetizing and leakage energy back to the supply (and thereby lowering the dissipation in the 'snubber' components). I suppose those might also be of benefit with PWM-ing a solenoid, but it's not an area I'm familiar with.

    Are you sure there is no other connection to the top of what looks like it has to be a current-sense resistor (the 0.22R)?

  • 0 in reply to jc2048

    Thanks for your reply, sorry for my absence from this thread. I am a bit over loaded now days. That circuit you posted sure looks close to what I have drawn. I don't have a board right now but three will arrive by weeks end and I will look more closely at the 0.22 resistor and connections to it. I thought it was just a current limiter but it will be best if I make certain.