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I'm playing around with some switching mode power supply things. For this reason, I'd like to switch the input voltage with a microcontroller, from the PWM output or other output pins.

The specifics:

  • Input voltage: 4V - 60V
  • Switching input signal: 3.3V (LVTTL)
  • Maximum switching speed: 50kHz - 1MHz (as high as I can easily get it)
  • Required current tolerance: 1.5A or 15W

There is no supply voltage besides the 3.3V signal.

I am looking for suitable components to do this as simply as possible. For me, simple means as few components as possible, and as few requirements on both the inputs and outputs of the components - I do not wish to impose any driving requirements to the microcontroller, or restrictions on how the switch can be used, or similar things. But I'd want the components to be suitable for a high efficiency switching mode power supply.

I know it can be done with a combination of n-MOSFET and a p-MOSFET and a couple resistors. I know it can also be done with a gate driver IC and a n-MOSFET, although I have been unable to locate one that would only require 3.3V supply voltage (and a boost pin?). I know I do not want a solution that would force some minimum off-time rate on the MOSFET to charge a capacitor, but I might be convinced otherwise. In theory, the most direct solution would be a 3.3V compatible solid state relay or analog switch, but the actual solid state relays available do not seem very suitable.

But, primarily I am just interested in learning more about the problem space here and the possible solutions.


Clarifying schematic stolen from another question:

enter image description here

  • Vin = 4V - 60V
  • Controller = 3.3V signal, toggling at, for example, 250 kHz
  • Q1 = the component, or the set of components I wish to find

So, strictly, the question is simply: what do I need in place of Q1?

(Note that this is just an example schematic, not exactly the sort of circuit that I wish to build.)

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    \$\begingroup\$ If the load is high side referenced (connected to 4-60V rail) then you can drive the bottom end with a N Channel MOSFet nly. The MOSFET can be driven by a gate driver. For speeds of up to around 100 kHz you can use two bipolar transistors and little else. If you want active high side drive you need the P Channel FET and a level shifter for drive but NO drive voltage above +ve rail. \$\endgroup\$ – Russell McMahon Jan 25 '14 at 14:30
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    \$\begingroup\$ I'm not quite sure what's the question here, a schematic would be helpful. Nevertheless, logic level power MOSFETs that can carry the 1.5A required are generally specified for (at least) 4.5V VGS operation and would not work properly with 3.3V. \$\endgroup\$ – realtime Jan 25 '14 at 17:43
  • \$\begingroup\$ I understand that this is a current output converter, but can you give some details on the output voltage range required? Are there further quantifiable constraints, e.g. maximum current drawn from the 3.3V rail, utilization of heat sinks, size, efficiency, etc. \$\endgroup\$ – realtime Jan 26 '14 at 5:31
  • \$\begingroup\$ I'd like to output around the same voltage range as in input 4 - 60V. I wish to try several different designs, so this is not exact. I'd prefer not to draw too much current from the 3.3V rail, preferably in the milliamp range, but I can probably work around that if necessary. \$\endgroup\$ – Nakedible Jan 26 '14 at 19:54
  • \$\begingroup\$ Is the 3.3V rail return necessarily the ground as shown in the schematic? Or is there some flexibility? Does the load necessarily have one side connected to ground? \$\endgroup\$ – Spehro Pefhany Jan 27 '14 at 15:53
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The 2N6782 N-channel FET is rated at 100v drain to source at a maximum 3.5A and 15W max power. I picked a through-hole part because the surface-mount part would be difficult to solder by hand.

To drive the FET, you can use the LT4440-5, which takes a logic level in and can drive an N-channel MOSFET switching up to 60 or 80V depending on the version. The supply current is negligible, only a few µA. The driver only comes in a surface mount part (SOT 23-6 or MSOP).

The only drawback is it requires a supply voltage (VCC) between 5 and 15V (thanks to the OP for pointing out this variant of the LTC4440 with a lower minimum supply voltage). Since you have only a 3.3v supply, you could use either a voltage doubler or a simple boost converter (such as the MIC2141) to supply the VCC voltage.

Both the driver and voltage doubler/boost converter should only require a few ma (if that) from your 3.3v rail.

The FET and driver should work up to 1 MHz, maybe a little higher.

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  • \$\begingroup\$ Great answer! When researching this myself I came up with the exact same conclusions: LT4440 is about the only part which seems to directly do what I want. However, the LT4440 seems to require a diode and a capacitor in addition to these for the boost PIN - so 5 components in total plus any needed bypass capacitors. Do you agree? \$\endgroup\$ – Nakedible Jan 27 '14 at 21:38
  • \$\begingroup\$ Forgot that the boost converter needs a switch and a coil as well, so 7 components. \$\endgroup\$ – Nakedible Jan 27 '14 at 21:53
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    \$\begingroup\$ @Nakedible Interesting you had found the LT4440 already. Choices for 60v operation are few. For supply voltages up to 30V, my favorite FET driver is the MIC5014. I think you meant the MIC2141 needs a diode and an inductor (not switch). So yes, seven components plus bypass caps. \$\endgroup\$ – tcrosley Jan 27 '14 at 22:39
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    \$\begingroup\$ Maybe I am missing a point here, but what is special about the LT4440 in comparison to other gate drivers, e.g. IRS21850 or FAN7171? These are also 3.3V compatible, can cope with the 60V input and need the same external bootstrap components. \$\endgroup\$ – realtime Jan 29 '14 at 5:49
  • \$\begingroup\$ @tcrosley I was referring to the "LT4440 is about the only part..." statement. It's my impression that in this application all drivers mentioned are equivalent, but I might be missing a point. \$\endgroup\$ – realtime Jan 29 '14 at 6:36
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What you're describing is digital control by the sounds of it - the micro is going to control the buck FET duty cycle to control the output, so it's going to require some sort of external sensing of the condition of the output as well as (possibly) computation and (definitely) programming of the duty cycle.

Do you plan to sample the output and do the feedback loop in software, or will you have an compensated error amplifier implemented in hardware and sample the error voltage only? This will dictate how much CPU horsepower you'll need, and if you'll benefit from DSP-like instructions (multiply-accumulate) and DSP-like features (like DMA).

One consideration is the precision of the duty cycle register. A coarse duty cycle adjustment capability can worsen limit cycle oscillation, where the output goes above and below the desired setpoint because the exact duty cycle needed isn't achievable (not an issue with analog PWM).

Another area you need to pay careful attention to is the maximum useful switching frequency your microcontroller will be able to generate. This will be dictated by several key constraints:

  • the PWM duty cycle register resolution (the minimum period must be a small fraction of the desired switching frequency)
  • the ADC sampling rate of your microcontroller
  • whether or not you're doing a full-fleshed controller in software

Why are these important?

  • Your switching frequency will need to be a fraction of the ADC sampling rate to avoid aliasing issues. You need at least one sample per period (more is better).
  • If you plan to compute a compensated loop in software (PID or other scheme) you'll need enough microcontroller horsepower to do the sampling and calculations each and every switching cycle, plus have leftover cycles for whatever else you need to do. If you plan to sample the output of a hardware error amplifier and do some simple computation to program the duty cycle, you'll take a lot of burden off the microcontroller.
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  • \$\begingroup\$ Thank you for the information. I appreciate it as the information is useful for the "next step" in my design, even though it doesn't really answer the question presented here. \$\endgroup\$ – Nakedible Jan 29 '14 at 19:46
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    \$\begingroup\$ I see it as part of the 'problem space' you takled about, albeit tangentally. \$\endgroup\$ – Adam Lawrence Jan 29 '14 at 20:15
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If I were you I would not bother using your own micro as there are so many products around that are able to do this for you.
Saying that Your design is missing all kinds of things like sense resistors for feedback of what the output voltage is to reduce error.

After all this I would recommend using the LT3724 from Linear Technologies. It has a switching frequency of 200kHz. They also give you full example schematics to use their IC which you could in theory swap out for your own micro controller. However I do not see why you would as it is so much easier to use their solution. I would recommend performing the simulation of it in SPICE to check that it meets your requirements.

Here is a link to the development board that has full schematics and BOM.

http://cds.linear.com/docs/en/demo-board-manual/dc736af.pdf

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  • \$\begingroup\$ I know! There's no point in doing this myself, I just wish to learn and to play around with different SMPS designs. And yes, the feedback side (and actually most everything else) is not in this question as the only thing I wanted to know are the various ways to drive a high side switch with 60V of voltage difference. \$\endgroup\$ – Nakedible Jan 31 '14 at 6:27
  • \$\begingroup\$ Also, just swapping out my own micro controller for LT3724 would not work exactly for the reason I opened the question. LT3724 has a built-in high side driver that can drive a 60V FET. A microcontroller does not. \$\endgroup\$ – Nakedible Jan 31 '14 at 7:05
  • \$\begingroup\$ Pick one of these drivers that will match the FETs that you decide to use. High Side Driver \$\endgroup\$ – Jonny Flowers Feb 2 '14 at 0:53
  • \$\begingroup\$ ...of those drivers, the only one which will go up to 60V is the LTC4440, which has already mentioned in the other answer, so no new information. \$\endgroup\$ – Nakedible Feb 3 '14 at 15:55

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