80v supply -> 12v output

There are plenty of 1amp SMT (223 and 252 package) regulators. The trick I am finding puzzling is getting the voltage from 60v+ down to an acceptable level (about 20v max) required by most regulators. I plan to use the voltage to drive the gate of paralleled MOSFETs to switch the entire rail on/off (for motor speed control).

The only solution I have so far is the zener/resistor method. The problem here is that I don't have enough resolution with the voltage swing and still maintain an acceptable current capability. Since the zener/resistor is always under load, a voltage of 60v vs. a voltage of

80v is a considerable difference in current consumption. I would also like to be able to use the circuitry with the voltage as low as about 14v, and still get 12v which would drastically reduce the current if the resistor was setup to handle 80v.

I guess I am searching for a stepdown regulator that will handle up to

80v, not eat a bunch of current, be small, and able to supply up to 1amp of power at roughly 12v.

The project calls for the 60v-80v supply to be able to power a PIC micro, LEDs, and the 10+v for the gates of a H-bridge motor control circuit (which uses P & N channel MOSFETs). The 60v-80v voltage is passed through the motor. The voltage source is multiple cells in series. The voltage needs to be dropped to a level that is suitable for a generic 7805 type of regulator, and also a 7812 type for the gate drives.

I figured that 1amp is overkill for the CPU, LEDs, etc. 500ma would probably be plenty, but I like overbuilding things. :)

makes DC-DC converters that *almost* cover the range you need (36-75v, $22). Might be worth poking around.

Hello Jim,

There are plenty. But you can't expect 1A in SMT under this scenario. The National LM5008 will only do a few hundred mA, no idea whether you could parallel some.

Why not use a controller and external FET? That's how I do those designs. You could either buy one with a built-in series regulator or drop down via resistor/zener to feed the chip.

Regards, Joerg

How are the MOSFETs configured to switch the entire rail on and off- where are they in the circuit IOW?

Hello Jim,

1A is no problem for low voltages but that's different above 35V or so. Not enough market I guess. Anyway, if all you want is to do is provide gate drive do you really need 1A? You only have to overcome the gate capacitance for switching and hold against the Cgd spike. After that no current is needed except a little for the drive circuitry and the logic parts.

Look at the duty cycle. Maybe you could hang a cap parallel to that zener to supply the charge spike.

Or do you want that line from your 60-80V rail to directly switch the FET? Then you could still use a threshold circuit down there that snaps the FET on or off, to provide clean switching.

Regards, Joerg

The Synqor PQ60120EML04 has a maximum input voltage of 100V (max operating is 80V), and will provide up to 4.2 amps at 12V with sufficient airflow.

Regards, Allan

It's not for a robot, and only one power source is available.

I am considering using a zener/resistor to power a PIC through a transistor that was "on" until the base was changed. The base could be controlled by the PIC and turned off when the switching power supply (generated and controlled by the PIC) was up and running. We definitely do not want excessive current consumption, which is what will happen with the zener/resistor type of voltage regulator at a higher voltage.

That's about 70W dissipation going linear .... not cheap with the heatsink, not SMT either.

An isolated step-down converter - if this is hanging off a nasty pulsed load, you might come to appreciate the isolation the will provide the ability to interface the way you want to.

If it must be SMT and it is a-few-off, rather than hundreds, the easiest & cheapest is to buy a module. 60 to 80 volts is close enough to telecom battery supplies to make something available. Mains switchers go down to that range too - maybe one can be found with the undervoltage designed low enough.

For CHEAP, no "wierdo components", I would use a self-oscillating current fed resonant Mapham inverter followed by a conventional linear regulator. I like those. But design time costs money too.

Sounds like this might be for a robot?

If you are running on battery power, why not consider a seperate smaller 12V battery for the control electronics?

I was going to go see if the LM317 comes in SMT (for a preregulator or so) and stumbled on this page:

Looks interesting. :-)

Good Luck! Rich

Reading between the lines of Joerg's first response ....

Why not a low voltage zener supply to a switchmode controller chip with an external FET or similar, to produce your 12V rail. Then run whatever reg you choose off the +12V to get your PIC/LED 5V supply. The only current load that needs to shunt the zener is the switchmode controller chip, NOT the PIC/LED load.

In article , Frithiof Andreas Jensen wrote: [....]

It may be cheaper to parallel devices. 70W of heat sinking can cost more than two 35W sinkings.

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kensmith@rahul.net   forging knowledge

Guess it would be too much to ask to bring a low voltage tap out of that "multiple cell" power source...

Look at the LM5010.

Best Wishes

Thanks.. I have looked at the LM5010, and that is even cheaper (and has a current rating of up to 1 amp). I have a slew of BLUE leds that require about 75ma each, so I need close to the 1amp figure.

Hello Jim,

Sure sounds like over-building. That costs money. Why not figure out a conservative power budget and add enough safety margin. I can't imagine a PIC with some LEDs tied to it consume more than 100mA. It would fry if it did. You might get away with a lone LM5008. Buck fifty or so in quantities.

Regards, Joerg

Hello Jim,

Careful, that one only goes to 75V. But maybe you could drop 20V or so with a large zener (watch its leakage though).

Regards, Joerg

I dont remember you saying whether this is a 1 off or a production run.

Linear v drops are going to be... 80-12=62 x 1A = 62W diss at least, very poor show for a battery project of any kind. So really you need a switcher. If cutting cost must be pursued aggressively, I'd look at an asynchronous 2 transistor sm reg. (switch mode, not surface mount) Used to make those for peanuts, you could scale one up a bit for 80v 1A.

With a basic unprotected async, you need to be able to control load surges, as the switching tr just stays on until the required V is reached, and your load startup surge, reservoir C and L all determine the overshoot. And of course the peak tr i. However since you're controlling the motor with a PIC it should be simple to keep the motor off during the initial power up. And when you turn the motor on, again use the PIC to keep it within the bounds of the psu. And if there are startup problems, soft start can be added easily enough.

NT