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shunt controller design

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IamSmooth

Sun Nov 30 2008, 04:32AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

I am beginning to think I don't have a good solution yet using FETs for shunting power. I want to keep the rail voltage 500v or less. As the voltage rises I want to shunt power away from an inverter. I see two solutions:

1. shunt some power to a dump load. I have 5 FETs. 25 ohm load. 5 ohm gate resistor. If I'm shunting 2.5A (0.5A per FET), does this mean that I am dissipating (500 - 25*2.5 - 5*0.5)*0.5A? This is over 200 watts per FET. This seems like a lot even with a heatsink. They are rated at 500W.

2. switch a relay that diverts all power to a dump load. Leave it on the load for a period of time and then check the line voltage. If it is under 500v the switch back; otherwise, keep it on the dump load. My problem with this is I will need a circuit consisting of IC chips to count a delay. These chips will require a 5v power source. I only have a variable rail voltage to use for power, so how do I get 5v from a power source that can vary from 0-500v?

As another idea, maybe I could use mechanical voltage controlled relays: one could monitor the mains voltage and switch to the shunt when mains power is gone; the other could divert to the relay when the voltage goes over 500v and switch back when it drops below 450v (numbers are arbitrary). If this is a good idea I need advice where I could get these (Mouser? Digikey?). If they do exist, the only problem I see is if the voltage stays within the hysteris band for an extended time. I would lose the power; however, if I have too narrow a hysterisis band I will stress the inverter as the shunt will prematurely switch back.

aonomus

Sun Nov 30 2008, 06:10AM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

Why not a comparator?

Dr. Dark Current

Sun Nov 30 2008, 08:53AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Well, relays are prone to failure, I would rather use FETs or IGBTs in switch mode instead of relays. Use several FETs, each driving a small bank of resistors, and turn on the desired number of FETs based on how much power you need to shunt.

Or do it the geek's way, let's say you use just 3 banks of resistors, each one larger by a factor of 2. With a clever digital controller, you can obtain 8 (2^3) steps of "regulation". With 4 banks it's 16 steps.

Sulaiman

Sun Nov 30 2008, 10:10AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

You may want to look at the system as a whole;

1) The source of your power is a mystery - I'll assume wind-hydro-solar power,
so the first place to consider is the source;
Hydro... throttle or divert the water flow
Wind... feather the rotors
Solar..shade the cells or rotate the collector
etc. etc.
These methods protect the power source as well as the inverter.

2) If you are producing power via a dynamo or alternator, can you control the field current?

3) If none of the above then I'd use pwm to switch a resistive load across the dc bus.
Only one load and one switching device required with continuous 'regulation'

Dr. Dark Current

Sun Nov 30 2008, 10:37AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Sulaiman wrote ...

3) If none of the above then I'd use pwm to switch a resistive load across the dc bus.
Only one load and one switching device required with continuous 'regulation'

Doh! How could I forget about the PWM

Of course this is the best way of dissipating the extra power if you can't regulate the source...

IamSmooth

Sun Nov 30 2008, 04:25PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

The source is wind, and there is a furling (feathering) mechanism. The shunt controller needs to be able to handle a situation in case the furling system fails, which it can since it is mechanical and subject to gusts.

How do I read up on PWM designs specific for my application.

aonomus

Sun Nov 30 2008, 08:40PM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

In the event of voltage going over 500V, depending on how fast you want your PWM (and depending on how much power you are generating) you may want to consider IGBT's cause of the heat-dissipation issues.

My only thought in terms of designing a PWM circuit is to use a ADC instead of a comparator, and use a uC to tell the FETs when to switch based on the input from the ADC.

IamSmooth

Sun Nov 30 2008, 10:25PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

How are IGBTs better than FETs? Are they more tolerant of heat. Here is a link to one I found that can handle 600v and 625W

Would I design the shunt circuit as if this were a bipolar transistor?

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