# LED diodes as both diode and load for boost converter

I'm doing an LED project with a Cree CXB3590 LED that I intend to underpower to experiment with efficiency and learn about boost converter design. I've made a 5uH inductor with 14 gauge magnet wire wound onto a T50 toroidal yellow/white(iron powder) core. I plan to use a 555 clock circuit and a 555 pulse generator with a horrible oscilloscope to determine the maximum pulse length for the inductor core and then experiment with frequency to compare total light output from the CXB3590 at different power levels to the output from single LEDs such as an XP-G or XR-E at the same power level. I'll be using PWM with a frequency limit ~1Mhz due to the iron powder core inductor. The large wire on the inductor and a decent MOSFET are intended to keep resistive losses as low as possible, hopefully negligible.

Because the CXB3590 is capable of using much more power than the boost converter circuit is likely to provide, I'm hoping to use it as both output diode and load, making the assumption that with each pulse the inductor will drive voltage up to the on voltage of the LED (~33-34V) and when that voltage is reached because the LED's effective resistance will become extremely low, it will not rise significantly higher as the inductor dumps its stored energy.

I'm hoping that this will result in the LED operating in a favorable efficiency band, operating in discontinuous mode and at close to the lowest voltage that will allow it to dissipate the power being pumped into it.

simulate this circuit – Schematic created using CircuitLab

So my question is this: For what reason(s) I would benefit from using the topology shown on the right, such as the inclusion of a capacitor helping clamp down the voltage the LED sees? the CXB3590 is rated at 86.4W, so at the ~3-10W I intend to operate it at, as I mention above it will likely operate discontinuously.

• Looks OK. In 2nd cct as cap goes up in value you will convert to continuous LED action with ripple. CCt 1 will always be discontinuous. At low frequencies cct1 will always flicker and cct 2 may or may not deep-ending on cap. – Russell McMahon Oct 8 '19 at 7:22
• LEDs have best luminous efficiency at low I. With pulsed I overall lumens will be lower - but not necessarily grossly so. If Pin is << PLEDmaxallowed all probably OK, but as P approaches PMax the pulses will cause power rating to be exceeded short term. Modern white LEDS have much lower Pabsmax/Pmax ratings than olde school LEDs or eg IR where high Ipulse is OK. "Fast diode" can be a FEt if desired to minimise voltage drop - but efficiency overall in this cct is liable to not be so marvellous than eg any Schottky of adequare rating will do. [I've used several million LEDs but far lower pwr]. – Russell McMahon Oct 8 '19 at 8:07
• Beware that your converter probably won't be terribly efficient. Or maybe even terribly inefficient. You're boosting by a factor of 20+ (assuming white LEDs), which is hard to do efficiently even with the best designs and components. Also, I suspect your target of 1MHz is pretty high, definitely experiment with lower frequencies. Finally, consider trying both topologies and measure which one performs better! – marcelm Oct 8 '19 at 23:33
• Inductor current will be continuous across the switching transition. Voltage will rise to whatever is required to ensure this. On an open circuit that would be high enough to transfer the current into the stray capacitance plus I^@R losses in wiring and cap ESR = VERY high. With the LED there I_LED initial = I_inductor_at_switch_open. | With 12V in and ~= 36V out -> Pinstantaneous ~= 3 x Pin at switch open time. At say 1A switch turnoff, PLED = 1A and VLED is set by LED I/V Vf curve. Current and voltage will fall from there. ... – Russell McMahon Oct 9 '19 at 0:56
• ... So LED POWER will peak at about 36V and fall across the inductor discharge so << 36W overall for 1A pulse. | At 1A in Iin mean is 0.5A or less (mean 0.5A during ton and 0 during toff with duty cycle of about 3:! at limit. So Pin About 6W at 1A so PLED less. So LED sees an about 36W pulse power for 6W DC in or 6:1+. So you can go to ABOUT 15W in without exceeding LED Pintypmax. Probably :-). YMMV. E&OE. – Russell McMahon Oct 9 '19 at 1:02

ADD: The left cct cannot run in CCM (continuous conduction mode) because the switch also shorts out the LED to 0V. But besides that now you have a low ESR path with a discharged cap that draws as much current as the inductor can swing V+/DCR and causes a high Q current resonant cct and adding pulses adds fuel to the fire with burnout on the series resonant circuit.

That's a serious flaw on the left. 1st it starts a low frequency resonance, the as the current builds up in the inductor the cap voltage is discharged at the same time. So the series diode switch is essential by lowering the Q switching the Cap off and ON. and allowing the voltage to accumulate with current switched pulses **

Examine the impedance of every part and try to choose parts towards 0.1% of LED ON resistance. (incremental V/I) to minimize losses. ( i.e. 2 mOhm ballpark )

Assuming 36V LEDs: (although you would be wiser to use 72V LEDs)

Rled = 1.7Ω= 1V/0.6A = 35.5V/2A - 36.5V/2.6A (slope)
Fast Power Schottky Diode: ~ 2 mOhm @ 20A STPS5045SG-TR 50A rated
FET Switch: <5m Ohm = RdsOn @ 10Vgs

Operate gate drive frequency at 10k to 50kHz max ( lower is more efficient)
Operate duty cycle < 75% pref. <50% ON time.

Choose large cores e.g. T50-26 = 0.5"D , AL= 33 nH/N² with 12 turns AWG16 magnet wire 3.2mΩ to get L = 33nH*240 = ~ 7.9 uH

Then with a very tight layout, attempt to achieve these current pulses in Falstad simulation.

LED Current = Cap Current (-ve ramp discharge) + Diode pulse current (+ve ramp charge)

Compute the efficiency from the floating power graphs with Average power.
+12V =-65.827 W
LED+ = 64.256 W
**loss = 1.571 W
/ 66W 100% = 2.5% @ 33kHz mostly in FET.
using low ESR , DCR, Rs (diode) components.

• I've been simulating a near identical circuit to the one you recommend and I've been learning a lot. I don't know how to simulate a not-purely resistive load in ltspice, but I managed to do basic voltage regulation by setting a clock pulse and having it skip pulses when output voltage is higher than desired and now I'm going to attempt current control while doing the same. You've been very helpful in terms of providing component values that will yield a functional hobby project design. There are 5 more days left on the bounty and I don't know if others will answer, – K H Oct 15 '19 at 2:19
• Yes . The left cct cannot run in CCM (continuous conduction mode) because the switch also shorts out the LED to 0V. So the diode switches the current path and while raising the voltage slightly ---with a ramp -- with sufficiently large C, low ESR CAP – Tony Stewart Sunnyskyguy EE75 Oct 15 '19 at 2:25
• You can export a Falstad shortcut link in answers but the shortcut domain only works in comments. – Tony Stewart Sunnyskyguy EE75 Oct 15 '19 at 3:13
• Logic gates and Op Amps have slew rate control but 0 Ohms out, so add when Ron when desired. FET's have RdsOn controlled by Beta only 20m is like 300 Ohms, 10 is like 50mOhm – Tony Stewart Sunnyskyguy EE75 Oct 15 '19 at 3:18
• TO make Falstad more real, add 0.05 ~ 0.1 Ohm ESR to the 220uF Cap as per datasheet and < 10 mOhm for a good choke. – Tony Stewart Sunnyskyguy EE75 Oct 15 '19 at 3:38

Your suggestion could work. I have done this with a single white LED where space and cost and simplicity were paramount. The boost diode wastes voltage and hence power. Your 33V string has about ten times the voltage that I had on yesteryears' birdsnest. The prospective power savings will not be great in percentage terms. You will waste more power in the LEDs because the current is very lumpy. The Ohmic losses in the LED bulk resistance will raise the LED junction temp. DC runs the LEDs coolest. Carefully look at what you are doing. The downside may be more than the upside making the design worse off.

• I'm interested to see how the experiment plays out. At 3W, an xp-g LED will be at 1.1A whereas the test COB will be at only ~90mA avg and the XP-G will be operating at a less favorable section of the luminous efficiency band as well as suffering higher $I^2R$ losses. You make a valid point though, the resistive losses from the tiny trace wires on the COB will likely add up. I know LEDs have much greater efficiency when underpowered, but I'm not sure how this plays out when barely kept at the turn on point. The cob and test LEDs will be mounted with arctic silver on oversized heat sinks. – K H Oct 8 '19 at 3:47
• Part of the test will be to adjust the COB and the test LED to similar light output and measure input power on both. I don't expect efficiency gains to justify the 10x-20x cost of the COB, but at least I'll have a COB to play with =). +1 for autism and helpfulness, but not definitive so I'll wait to see what others have to say. – K H Oct 8 '19 at 3:50
• WARNING: Yellow and White core colour is a Micrometals trademark (really). If your core IS micrometals then it will meet spec sheet well. IF it is Asian and non micrometals then it may meet some specs. One of the specs which doesn't bother you here is lifetime. With heating the core ages and the binder fails and the core particles separate and core losses get higher and it gets hotter still and .... . Who would have thought that an iron powder core can suffer thermal runaway (at sloth speed). | I had buck converters made in Taiwan for a client. Spec said MUST use micrometals cores. .. :-) – Russell McMahon Oct 9 '19 at 1:06
• @RussellMcMahon Provenance of the cores is aliexpress, so it is not certain they are cores, let alone iron or powder =). They do look toroidal though. Hopefully they took a good look at the product they were knocking off if they're(likely) not real product. There is no comparable storebought core to compare to, so I'll have to experiment to see how well they perform. Where I'm using questionable parts, I'm attempting to use them below their expected spec. I tried to find a reference to pre-calculate core saturation, but couldn't find formulas, let alone that I could understand. – K H Oct 9 '19 at 1:44