• The modification

benjaminaigner04/02/2017 at 08:05 0 comments

It's very easy to modify this converter to be constant-current controlled:

You need following parts:

• 1k potentiometer
• 3W shunt (the value depends, first setup: 0.86Ohm)
• 2pin Screw terminal
• A little bit of 1.5mm² wire

As you can see in the schematic image, not very much to do.

If you need further help, please write :-)

I tested this setup already with following currents and LED power:

• Vin: 12V Iout 1050mA, 2x10W LED
• Vin: 12V Iout 1750mA, 1x50W LED

You can adjust the current with the poti, if it has the lowest resistor, the current is also the lowest possible one. Due to the FP1539 feedback circuit, the controller regulates until there are 0.5V at the feedback pin.

If you have a 0R poti, this eqals to:

I = U / R = 0.5V / <R-shunt> = 0.5V / 0.68Ohm = 0.735A

Of course, a lower shunt value increases the minimum current, but it also decreases the power losses on the resistor:

Pshunt = I out ² * Rshunt = 1.75A² * 0.68Ohm = 2.1W -> nice, within the specs :-)

If you want to increase the efficiency, decrease the Rshunt value.

For example: If I know, that I only use about 1.5A (or 1.75A+/- a little bit adjustment), let's try different values:

I = U/R = 0.5V / 0.47Ohm = 1.06A min -> Pshunt = 1.75² * 0.47 = 1.43W (only 70% of the previous value)

next value

I = U/R = 0.5V/0.33Ohm = 1.51Amin -> Pshunt = 1.75² * 0.33 = 1.01W (even more efficient)

Ok, for my setup, I will modify my CC controller to support only 1.5A+, but it reduces the wasted heat by 1W (2% :-)).

Next time, I will write about the whole feedback circuit in detail!

• The main DC/DC converter

benjaminaigner04/01/2017 at 13:20 0 comments

The heart of this module is a FP5139 Asynchronous Boost Controller.

It's packaged in TSSOP8 and has different features we have to deal with:

• Soft-Start function
• Short Circuit protection

Basically, this booster module is designed like the Application Information in Figure 14 of the FP5139 datasheet.

Some minor modifications compared to this app note:

• Different Feedback resistors
• Voltage limiter (FP5139: Vin 1.8-15V)

If you look at the picture, following colors correspond to these parts of the app note:

orange: FP5139 chip

yellow: Q1/Q5 totem-pole / push-pull gate driver

pink: MOSFET & Diode (Q2/D2)

blue: voltage limiter (some kind of transistor circuit)

green: feedback circuit filter capacitor and R6 in the app note (lower side of a voltage divider)

turquoise: oscillator settings & soft-start capacitor

red: feedback pin, that's what we need

BTW: The MOSFET was destroyed by me (while removing the heatsink), so your boost module might look different :-)
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