DIY Scalable Bench Power Supply Design

[void_error]

If I'm going to use high side current sensing I might as well use a 'regular' 5V rail-to-rail opamp with low offset and a P-channel MOSFET like in one of the application notes on high side current sensing. I'll probably replace the resistor with a curent sink and the zener with a TL431. It'll still be simpler than auto-ranging and won't require any 'special' components which was the initial goal of this design.

[David Hess]

I'm agree. More parts make frequency compensation difficult. Do you have any reference to discrete solution when we comes to current monitoring?

Besides using the methods that the mentioned Tektronix and HP power supplies use for their current limiting, this application note from Analog Devices pretty much covers it:

http://www.analog.com/library/analogdialogue/archives/44-12/high_side.html

I favor the first circuit shown above but the problem with it is that at low output voltages, the current monitoring circuit runs out of compliance if the output is referenced to ground.  I would reference the output to a point below ground to avoid this and level shift the control signal if necessary.

Yes, separate floating bias supply add some parts but I'm still fine with it if overall result is good.

The solution I like for a simple separate floating bias supply is to include a small and inexpensive auxiliary transformer.  Split bobbin power transformers like the Hammond 229, Pulse LP, and Triad FP series have high isolation and low capacitance if that matters.  They are inexpensive, small, and have dual primary and secondary windings so they support 120/240 VAC inputs and provide a center tapped output or dual outputs if needed.

[void_error]

Besides using the methods that the mentioned Tektronix and HP power supplies use for their current limiting, this application note from Analog Devices pretty much covers it:

http://www.analog.com/library/analogdialogue/archives/44-12/high_side.html

I favor the first circuit shown above but the problem with it is that at low output voltages, the current monitoring circuit runs out of compliance if the output is referenced to ground.  I would reference the output to a point below ground to avoid this and level shift the control signal if necessary.

That's exactly the circuit I was thinking about, having done a lot of research on high-side current sensing. The last version I've been working on allows easier substraction of the base drive current from the output current which means I can move the current sensing circuitry before the pass transistor, at least 5V above ground.

[dannyf]

The use of p-ch mosfet vs. pnp is an interesting trade-off. A pnp allows the sensor to work at lower voltage differential; HOwever, because of base current, the error rate is higher; The use of a p-ch minimizes that current error but doesn't work at as low of a voltage differential.

[David Hess]

That's exactly the circuit I was thinking about, having done a lot of research on high-side current sensing. The last version I've been working on allows easier substraction of the base drive current from the output current which means I can move the current sensing circuitry before the pass transistor, at least 5V above ground.

I think one of the more recent Agilent power supplies did exactly this by mirroring the base drive to the power transistor and adding it to the measured current.

[David Hess]

The use of p-ch mosfet vs. pnp is an interesting trade-off. A pnp allows the sensor to work at lower voltage differential; HOwever, because of base current, the error rate is higher; The use of a p-ch minimizes that current error but doesn't work at as low of a voltage differential.

If high voltage JFETs were still available, I would consider one of them.  I was going to suggest a p-channel depletion mode MOSFET but I could not find any.

[prasimix]

Besides using the methods that the mentioned Tektronix and HP power supplies use for their current limiting, this application note from Analog Devices pretty much covers it:

http://www.analog.com/library/analogdialogue/archives/44-12/high_side.html

I favor the first circuit shown above but the problem with it is that at low output voltages, the current monitoring circuit runs out of compliance if the output is referenced to ground.  I would reference the output to a point below ground to avoid this and level shift the control signal if necessary.

Excellent article, thanks for recommendation. Yes, first one has a real issue with low output voltages. Second and third cannot be used for low output currents.

[prasimix]

That's exactly the circuit I was thinking about, having done a lot of research on high-side current sensing. The last version I've been working on allows easier substraction of the base drive current from the output current which means I can move the current sensing circuitry before the pass transistor, at least 5V above ground.

I think one of the more recent Agilent power supplies did exactly this by mirroring the base drive to the power transistor and adding it to the measured current.

Do you possibly have anything to share with us (a model name or circuit block diagram or schematic)?

[prasimix]

The solution I like for a simple separate floating bias supply is to include a small and inexpensive auxiliary transformer.  Split bobbin power transformers like the Hammond 229, Pulse LP, and Triad FP series have high isolation and low capacitance if that matters.  They are inexpensive, small, and have dual primary and secondary windings so they support 120/240 VAC inputs and provide a center tapped output or dual outputs if needed.

What's about "quiet" hybrid bias supply, like solution presented (or marketed) here? In that case you can avoid auxiliary transformer and use the same Vin as main power loop. Yes, it's cheaper then switching regulator + 2 LDO's but possibly require smaller PCB space and do not need 115/230VAC to be present on the PCB.

EDIT: bad link reported by David Hess is corrected.

[David Hess]

That's exactly the circuit I was thinking about, having done a lot of research on high-side current sensing. The last version I've been working on allows easier substraction of the base drive current from the output current which means I can move the current sensing circuitry before the pass transistor, at least 5V above ground.

I think one of the more recent Agilent power supplies did exactly this by mirroring the base drive to the power transistor and adding it to the measured current.

Do you possibly have anything to share with us (a model name or circuit block diagram or schematic)?

I just remember studying the schematics in connection with a discussion on the EEVblog forums months ago.  I think it was one of their recent programmable power supplies.

The solution I like for a simple separate floating bias supply is to include a small and inexpensive auxiliary transformer.  Split bobbin power transformers like the Hammond 229, Pulse LP, and Triad FP series have high isolation and low capacitance if that matters.  They are inexpensive, small, and have dual primary and secondary windings so they support 120/240 VAC inputs and provide a center tapped output or dual outputs if needed.

What's about "quiet" hybrid bias supply, like solution presented (or marketed) here? In that case you can avoid auxiliary transformer and use the same Vin as main power loop. Yes, it's cheaper then switching regulator + 2 LDO's but possibly require smaller PCB space and do not need 115/230VAC to be present on the PCB.

Your link does not work for me but there is nothing wrong with generating the floating bias supply directly.  Switched capacitor techniques can even be used to avoid magnetics.  I would probably use a small high frequency inverter with an isolation or pulse transformer because the noise is easier to control than with a switching regulator.  High frequency sine wave drive is quieter and easier to filter yet.

[prasimix]

Your link does not work

Sorry, now it should works.

[prasimix]

The use of p-ch mosfet vs. pnp is an interesting trade-off. A pnp allows the sensor to work at lower voltage differential; HOwever, because of base current, the error rate is higher; The use of a p-ch minimizes that current error but doesn't work at as low of a voltage differential.

If high voltage JFETs were still available, I would consider one of them.  I was going to suggest a p-channel depletion mode MOSFET but I could not find any.

If 40V is high enough then there is a possibility to get some LSJ74 following this announcement.

[David Hess]

Your link does not work

Sorry, now it should works.

I was thinking hybrid like in integrated versus hybrid instead of a switching pre-regulator combined with a linear post-regulator.  There are three problems with what is described in the article you linked:

• Linear regulators have line rejection which falls with frequency.  If you want to keep the noise out, then you need to passively filter it anyway so why include the linear regulator if it is not needed?
  
• It is complicated compared to a charge pump or inverter.
• The outputs are not floating.

With careful design the noise from an isolated switching regulator will not be a problem.  It is just easier to get low noise out of an inverter which is why I suggested it.  Transformer flux is low so flux leakage is attenuated and the frequency and harmonic components are relatively fixed making them easier to filter.  If sine wave drive is used then harmonics are a non-issue.

If you do not want to wind your own transformer, they make standard transformers for this type of thing if you know where to look.  Pulse transformers designed to provide isolated base or gate drive work well also.

http://www.coilcraft.com/lpd5030v.cfm
http://www.coilcraft.com/prod_isolation.cfm
http://www.coilcraft.com/prod_gatedrive.cfm

[David Hess]

The use of p-ch mosfet vs. pnp is an interesting trade-off. A pnp allows the sensor to work at lower voltage differential; HOwever, because of base current, the error rate is higher; The use of a p-ch minimizes that current error but doesn't work at as low of a voltage differential.

If high voltage JFETs were still available, I would consider one of them.  I was going to suggest a p-channel depletion mode MOSFET but I could not find any.

If 40V is high enough then there is a possibility to get some LSJ74 following this announcement.

They used to make JFETs with Vds ratings above 100 volts.  A p-channel high voltage depletion mode MOSFET would be ideal because of the low Vgs but it is not really necessary.  A p-channel enhancement mode MOSFET will also work fine in this application as long as the bias supply can provide the higher voltage for the gate drive which should not be a problem at all.

[Marco]

With careful design the noise from an isolated switching regulator will not be a problem.

Why isolated? Why not just go say 1V below the bus voltage and boost from there? (You can buy single cell to 5V boost modules dirt cheap.)

[prasimix]

With careful design the noise from an isolated switching regulator will not be a problem.  It is just easier to get low noise out of an inverter which is why I suggested it.  Transformer flux is low so flux leakage is attenuated and the frequency and harmonic components are relatively fixed making them easier to filter.  If sine wave drive is used then harmonics are a non-issue.

I suppose that you are suggesting inverter only for negative bias supply connected to the output of positive bias supply or to the main rectifier.
Using sine wave instead of square has obvious advantage. Does some of inverters or low power switcher widely available use sine wave? Thanks for Coilcraft recommendation. I'm aware of their offer, they have some really nice items.

[David Hess]

With careful design the noise from an isolated switching regulator will not be a problem.

Why isolated? Why not just go say 1V below the bus voltage and boost from there? (You can buy single cell to 5V boost modules dirt cheap.)

I thought we were discussing how to use a floating bias supply in connection with the control circuits of a higher output voltage power supply.  This is a simple but sometimes confusing way to avoid the problem of using 5, 15, or 30 volt operational amplifiers to control a power supply with a 30 volt or higher output.

I suppose that you are suggesting inverter only for negative bias supply connected to the output of positive bias supply or to the main rectifier.

I was suggesting an alternative to using a separate 60 Hz power transformer to produce a bipolar floating power supply for the control circuits of a high voltage power supply.  If you were winding your own transformer or just added a second transformer, you could have a pair of floating supplies from the same inverter.

Using sine wave instead of square has obvious advantage. Does some of inverters or low power switcher widely available use sine wave? Thanks for Coilcraft recommendation. I'm aware of their offer, they have some really nice items.

Low noise inverters may use sine wave drive.  Some low noise switching power supplies use quasi-resonate operation which amounts to the same thing and has the same advantage.

I linked the Coilcraft examples because they have good availability from distributors.  Other companies make similar products.  Few people want to wind their own.

[Marco]

I thought we were discussing how to use a floating bias supply in connection with the control circuits of a higher output voltage power supply.  This is a simple but sometimes confusing way to avoid the problem of using 5, 15, or 30 volt operational amplifiers to control a power supply with a 30 volt or higher output.

Oh ... why not just use some diodes and emitter followers for that? I thought you wanted to use the separate power supply so you didn't need to keep as much headroom from the bus voltage (boosting a little on top would give you headroom for the control circuitry, wildly inefficient of course but the control circuitry doesn't use much static power with a FET pass transistor).

[David Hess]

I thought we were discussing how to use a floating bias supply in connection with the control circuits of a higher output voltage power supply.  This is a simple but sometimes confusing way to avoid the problem of using 5, 15, or 30 volt operational amplifiers to control a power supply with a 30 volt or higher output.

Oh ... why not just use some diodes and emitter followers for that? I thought you wanted to use the separate power supply so you didn't need to keep as much headroom from the bus voltage (boosting a little on top would give you headroom for the control circuitry, wildly inefficient of course but the control circuitry doesn't use much static power with a FET pass transistor).

That is essentially what it amounts to.  It is just another way to go about it.

[prasimix]

I was suggesting an alternative to using a separate 60 Hz power transformer to produce a bipolar floating power supply for the control circuits of a high voltage power supply.  If you were winding your own transformer or just added a second transformer, you could have a pair of floating supplies from the same inverter.

Ok, so what about solution like one in the attachment? Of course it could be without LDO's on output but just with proper filtering.

EDIT 2015-02-26: A tested solution is presented here.

[David Hess]

I was suggesting an alternative to using a separate 60 Hz power transformer to produce a bipolar floating power supply for the control circuits of a high voltage power supply.  If you were winding your own transformer or just added a second transformer, you could have a pair of floating supplies from the same inverter.

Ok, so what about solution like one in the attachment? Of course it could be without LDO's on output but just with proper filtering.

The point is to generate a floating supply relative to the output and not the ground.  For that the ground and input connections of your switching regulator and LDOs would be reversed.  If you wanted a negative bias supply below ground, then a third winding would be needed.

[prasimix]

I was suggesting an alternative to using a separate 60 Hz power transformer to produce a bipolar floating power supply for the control circuits of a high voltage power supply.  If you were winding your own transformer or just added a second transformer, you could have a pair of floating supplies from the same inverter.

Ok, so what about solution like one in the attachment? Of course it could be without LDO's on output but just with proper filtering.

The point is to generate a floating supply relative to the output and not the ground.  For that the ground and input connections of your switching regulator and LDOs would be reversed.  If you wanted a negative bias supply below ground, then a third winding would be needed.

Thanks for correction. I'd like to skip for now usage of third winding or aux transformer and try to do everything with main transformer.

[prasimix]

I thought we were discussing how to use a floating bias supply in connection with the control circuits of a higher output voltage power supply.  This is a simple but sometimes confusing way to avoid the problem of using 5, 15, or 30 volt operational amplifiers to control a power supply with a 30 volt or higher output.

Oh ... why not just use some diodes and emitter followers for that? I thought you wanted to use the separate power supply so you didn't need to keep as much headroom from the bus voltage (boosting a little on top would give you headroom for the control circuitry, wildly inefficient of course but the control circuitry doesn't use much static power with a FET pass transistor).

Thanks Marco for suggestion. I think that we are talking here about two different things. My initial conversation with David Hess was about bias power supply for control circuit op amps with split rail (i.e. +15/-15V) what is not used in void_error design. My main question is how to generate negative voltage without deploying additional winding or aux transformer.
Suggested bootstrapping could be beneficial to drive serial BJT/FET for what void_error used so far additional driver circuit (U4, Q3-Q7).

[prasimix]

That's exactly the circuit I was thinking about, having done a lot of research on high-side current sensing. The last version I've been working on allows easier substraction of the base drive current from the output current which means I can move the current sensing circuitry before the pass transistor, at least 5V above ground.

I think one of the more recent Agilent power supplies did exactly this by mirroring the base drive to the power transistor and adding it to the measured current.

Does something like this make sense? Current monitor is located before serial BJT/FET so main current + serial regulator current are measured and available on the LTC6102 output. To remove serial regulator current from the measurement, two additional op amps is used: one to measure voltage drop on R11 that is delivered inverted to the IC1A and subtracted from LTC6102 output voltage. Resulting value is then delivered to the current control "logic".

EDIT: LT6102 corrected to LTC6102

[dannyf]

Current monitor is located before serial BJT/FET

I am not sure why you want to do that. The rest of the circuit could consume considerable current, rendering a bigger error before the current sense amplifier's reading and true load current, not to mention the tougher common mode signal at the higher end.

I would put the current measurement after the regulator, and power the current sense amplifier before the regulator.