Help desiging an offline led strobe driver

[mrweathers]

Hey there! first post here.
I've been working with LED strobe lights for a few months now and I'm looking to upgrade my system.

Currently, my system has 3 separate modules:
1) DC power supply: I have a 12V supply and a 90V low current boost converter for different light sources
2) Strobe Controller circuit
3) Teensy based pulse generator

The strobe control circuit is as simple as can be for now:
Use a mosfet as a low-side switch on the LED.


This means my LEDs are voltage driven instead of current driven.  This has been fine while working with resistor laden LED strips, and even a couple cheap COB LEDs.

But now I have a new list of requirements:
1) Run of rectified mains: cheaper and less bulky than DC supplies
2) Adjustable current limit: In the case that my strobe pulses are too long, I don't want to burn out my LEDs
3) Strobe pulse width control with ~5us resolution
4) current limit up to 4A
5) Ideally, but not necessary: have a different current limit for fast pulses and slow pulses

I see 2 paths forward and I'd like to discuss the merits of each path before proceeding.

Path 1) All analog: Use an op-amp and current sense resistor to apply linear feedback to the mosfet gate.  Apply strobe control pulses to the op-amp inputs
Advantages:
              *) I've used this circuit topology before, and its easy to understand
disadvantages:
              *) Need to generate a low-voltage supply rail for the op amp
              *) mosfet dumps a lot of heat when its in linear mode

I'm sure this would work, but I'm tempted to take a more sophisticated route.
After disassembling some LED spotlights, I found a class of integrated circuit that uses PWM banging on the mosfet gate regulate the current.   Some of them even integrate the mosfet, run directly off rectified mains, and have fancy power factor correction features (which I am not interested in for now).  My strobe pulses are non periodic, so I can't just use a typical PWM interface at a fixed or variable frequency.

This leads me to path 2:

Path 2) Use an offline led driver circuit such as AL9910SP: https://www.digikey.com/en/products/detail/diodes-incorporated/AL9910SP-13/2639280
Advantages:
         *) Runs directly off rectified mains
         *) External PWM control pin for applying strobe pulse
         *) Analog dim pin to set different current thresholds
Disadvantages:
         *) New chip to work with, may take a few tries
         *) I'm using the chip a bit off-label and there aren't too many resources to consult on this usage.  For example, I may need to modify the LC network on the load for my use case.

My thoughts on how to adapt the AL9910 for my use case:
*) Operate at the highest PWM frequency (~300kHz)
*) Use an RC network at R_sense to get a different current limit for fast and slow pulses.
*) Omit the inductor L1, or choose a much smaller value, to get fast turn-off

Any thoughts on this topic?
Other IC suggestions?
Is my description coherent enough to discuss?

[moffy]

My principle concern for using the AL9910 directly from the mains would be safety. All the connected LEDs would be live. What is the maximum voltage you require? I just bought a pair of 60V/5A power supplies for under $200 AUD, very light and easy to use CC or CV. Perhaps if you purchased a suitable supply and just concentrated on the strobe switching?

[mrweathers]

Thanks for the reply moffy

Yeah there are always safety issues when working with mains.  But the system will be sealed in a grounded enclosure.  I am repurposing a commercial LED floodlight enclosure.  The only exposed port will be the PWM (and maybe LD) pins on the driver.

I'm going to be building at least 10 of these and setting them up at events, so using the a mains power outlet is ideal.  Not just for reduced cost, but the small size as well.

[moffy]

Thanks for the reply moffy

Yeah there are always safety issues when working with mains.  But the system will be sealed in a grounded enclosure.  I am repurposing a commercial LED floodlight enclosure.  The only exposed port will be the PWM (and maybe LD) pins on the driver.

I'm going to be building at least 10 of these and setting them up at events, so using the a mains power outlet is ideal.  Not just for reduced cost, but the small size as well.

You realise that if anyone is injured you could be sued apart from having been involved in someone's injury? There are safety certifications and standards that you need to apply especially for mains operated equipment. I am not sure about the USA but in Australia things can get quite sticky pretty fast.

[Terry Bites]

Use a supply with isolation when ever possible, your life is worth than few bucks saved.

[Zero999]

There is nothing wrong with directly running off the mains. Large xenon strobes would often work at even higher voltages than the mains, via a voltage doubler or step-up transformer. A long string of LEDs will be able to work directly off the mains, much more safely, than an old fashioned strobe, because a massive capacitor isn't required.

The most important thing is adequate separation between the user and any control circuitry is essential for a circuit directly connected to the mains. It might work out cheaper to go with a switched mode power supply which solves that.

What sort of power/energy/duty cycle are to looking to run it at?

[mrweathers]

I am basing the design off this kind of commercially available LED floodlight, which is also powered directly off mains.

The most important thing is adequate separation between the user and any control circuitry is essential for a circuit directly connected to the mains. It might work out cheaper to go with a switched mode power supply which solves that.

The entire device is enclosed and grounded.  The only way for someone to get a shock is to break the glass and touch the pcb.
In fact, I'll be using that exact enclosure.  Swapping their LED board with my own strobe controlled board.
Aside from the choice of LED chip and driver, the only real difference between the commercial product and my own is the PWM input and/or linear dimming input.  I plan to expose this input by drilling a hole in the enclosure and fitting it with a 3.5mm audio jack feedthrough.

I can use an optoisolator on the input signal to keep it completely isolated from the rectified mains used to power the LEDs.  This costs <$5 in components per device.

What sort of power/energy/duty cycle are to looking to run it at?

I want to operate in continuous mode or strobe mode, but I want to cheap out and order only 1 pcb design.
Thats why I suggested using an RC network for R_sense to set a different current limit for short pulses.
I will typically operate at <10% duty cycle in strobe mode, roughly 1ms pulses @ 100Hz.
In continuous mode, I am aiming for ~100W, which would give me 3000-8000 lumens depending on the color.

Use a supply with isolation when ever possible, your life is worth than few bucks saved.

When I am building and testing the device, I will power it with an isolation transformer and a variac.   But I don't intend to use one once everything is sealed up and the device is in normal operation.

You realise that if anyone is injured you could be sued apart from having been involved in someone's injury? There are safety certifications and standards that you need to apply especially for mains operated equipment. I am not sure about the USA but in Australia things can get quite sticky pretty fast.

I'm not selling these devices or using for a commercial purpose, so I don't think certifications and standards apply.  I'd love if my toys were good enough to sell, and I will welcome the hurdles of certification if I ever get to that point.