# DC-DC Converter

• posted

Hi All,

Another learning curve for me... Hope can seek enlightenment.

I am searching for a DC/DC converter; input 60V, output 48V for a 5KW power.

Am I missing something here since this should be more of electrical and I am an electronics person.

Is there such a component because the highest power I found is 700W.

Some background:

I have a unregulated DC source (48VDC to 60VDC; 5KW) but I need a constant 48VDC;5KW output... Hence I thought of a dc/dc converter or voltage regulator. It seems that I am heading the wrong direction....

• posted

Is the (-) source isolatable? In other words, the - output of this source is not commonly tied to another common.

You can use a row of high current MosFets being driven from a PWM signal. The drains on the fets will be the low side of your supply through a heavy duty inductor so that we can achieve a smooth output. You don't want the PWM to be visible in your final DC and the inductor becomes part of the regulation and filtering equation.

If you want to do the bipolar analog regulating root, you can do that too. You will need a good heat sink that can hold at least 10 or more power transistors rated for 120 watts or better.

At 48 volts out at 5kw that is ~ 104 amps of power. You have the range of 48 to 60 Volts to work with, this gives you a 12 volt window across the trannies.

12*100= 1200 watts of dissipation in the transistors. Lots of energy loss there from heat but it is doable.

If you go the Mosfet way and use pulse width modulation in line with an inductor, you can then smoothing regulate that.. Heat lose will be low. That is because the Fets turn on with very low R. R(On), lots of current can flow through them with out heating.

For example, an average power MosFet is ~ 0.010 ohm when fully turned on.

P = I^R = 100^*0.0100 = 100 watts

Of course you don't want to force a single unit to do this work. Over all package temperature must be keep down.

Use a few high current, high wattage units in parallel. Driven them from a square wave PWM source that has regulated duty cycle to adjust output voltage via a low side common source configuration.

This should give you something to think about.

Jamie

• posted

Thanks Jamie for opening another area ... I will find up along this train of thoughts.

Output may not be isolatable but I will relook at the design again.

Btw, what about rectifying from generator output? Meaning, a 5KW generator set outputs AC; to convert this to DC, there must be a "big" enough rectifier.

I had seen a couple of AVR (auto voltage regulator); but the wires and connections to it does not seems capable of handling 100A.... I guess they are more for regulating but even so there must be something missing.....what is a high power LM237 ??

Generator ----> ??? ----> Regulated DC output

• posted

pardon my last post on the "Generator ----> ??? ----> Regulated DC output " ... found a couple of AC to DC rectifier.

• posted

what kind of generator is this? being an AC output, it must be an Alternator? If so, you can control the field to regulate the generator. That requires very little current. something that can be done with a small single driving transistor.

As for DC out, a large bridge rectifier is needed here.

Most likely better off using some stud mounted singles to make up a bridge. or some plastic heat sink mountable types.

Jamie

• posted

You cannot get 5KW regulated output with only 5KW unregulated input. There will always be some loss. You need to either increase the available power input or lower the regulated power output requirement.

With a DC-DC converter, Input_power = Output_Power/efficiency. Say you find a DC-DC converter that is 90% efficient. At 5KW output, you'd need at least 5556 watts input power.

With a linear regulator, Input_Power = Output_Power + Wasted_Power and Wasted_Power = Vin-Vout * Current. Your Vin is 60 volts worst case and your current is Pout/Vout = 5000/48 = 104.17, so Wasted_Power = 1250 watts.

The "analog" (the proper name is "linear") regulator mentioned in one reply as something that could be done is not worth considering. You'd need over 6250 watts input, and have a huge amount of heat to dissipate, as was mentioned. On top of that, if the input voltage dropped down to 48V, the output would be lower than 48 volts. Linear regulators always need Vin to be higher than Vout.

Ed

• posted

if you regulate the (-) side with a low side common emitter(NPN), that drop isn't so bad that way.

Still not as good as using MosFets. :)

Jamie

• posted

I need a smaller one: 12->~40V at a few amps.

Anyone seen such surplus?

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• posted

12V to 40V ??
• posted

yes; 12V [or 13.8...] to about 40VDC.

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• posted

Saw this: