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Modding a boost converter for low voltage

A project log for Boost converter for low voltages

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K.C. LeeK.C. Lee 5 days ago0 Comments

I modded a LM3478 (3V to 40) eval board for low voltage boost.  With the modifications, I am hoping it would work for lower voltages once the output is running at 5V.

There are a few changes:

Part list

Q1: I am using Si7806ADN (30V, 12A, 0.013R).  It is just a small N-MOSFET that I have - nothing spectacular.

L1: Coilcraft DO3316T-223.  There are 22 turns on the core.  I made a tab at the 7th turn.  7T:14T

This is the old version of the eval board when National Semiconductor was still its own company.

To create small island of copper fill on the PCB for the mods, I use Polyimide tape (aka Kapton tape) for insulation and overlay small piece of copper tape.

Load test:

The bench supply was initially set to 3.5V for bootstrapping the circuit.  Once the output is running at 5V, the voltage is dialed back until it falls out of regulation. A bootstrap circuit could be used to provide this voltage instead.

The lowest input voltage for this load is 0.45V.  My bench supply doesn't have enough resolution, so I hook up my DMM to read the input.  Smaller loads requires lower voltages.  

The load is 70.2R drawing 5.05V at 72mA.  The efficiency is (5.05^2/70.2R)/(0.45*1.37) = 58.9%

I can probably play with the inductor turns ratio and the MOSFET to try to get higher power.

The following scope trace shows the gate drive to the MOSFET.

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 I did a LTSpice simulation with a slightly different part: LT3758, but with same power components.  The simulation thinks it can supply 250mA, but that doesn't happen here.

I made some changes trying to get more power, but it isn't possible.  The changes allows the same load to run from slightly lower voltage 0.376V.  It means it can run from a single solar cell under non-ideal condition.  

(5.05^2/70.2R)/(0.376*1.70) = 56.8%

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It turns out that the 1206 0R (Rsn) wasn't quite 0R.  I measured 40mV drop across the "0R" jumper, so there was some resistance there.  I have replaced it with a wide piece of copper foil and the boost converter now supports a 33R (36.2R) load (140mA) at 5.05V from a 0.449V source. The results now agrees with my LTSpice simulation.

Efficiency = (5.05^2/36.2)/(0.449*2.31) = 67.9%  which is a big improvement from the sub 60% for only half the load.

It is charging my ipod from USB.

Here is the eval board after all the new mods. It is cleaner too.

The "0R" resistor increased the voltage drop across Q1 fooling the PWM circuit to limit the duty cycle which limits the power.  As the resistor is remove, there is an issue going below 0.42V with this load as the PWM duty cycle increases and current drawn beyond my 5A bench supply. 

I'll need to adjust the 470R series resistor into Isense pin as that can be used to inject a DC offset to limit inductor current.

Inductor current peaks around 4A for the 150mA load at 0.4V in LTSpice simulation (file here) .  I pick 5A as a limit. The duty cycle starts to drop below 100% when Current sense >75mV.

Q1 RDS(on) = 0.0054R,  0.0054R * 5A = 27mV as the target.  By setting RSL = 1K, VSL adds an additional DC offset of ~50mV and that makes 75mV.   The PWM controller would start to reduce duty cycle when the inductor current starts reaching 5A.

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The majority of the current flow through the 5.5 turns winding of the tabbed 22uH inductor.  The inductance value is around 1.5uH, the saturation current for the same magnetic core is good for 7-8A.  

They used AWG 26 for the 22uH inductor.  I rewinded the first 5.5 turns with AWG 24 as that's where the maximum current flows when the MOSFET is on.   The remaining turns sees less than rated current.  There is a limit on the copper wire thickness as the secondary windings need to be on too., so AWG24 was a reasonable compromise.

Sadly, the reduction of the DC resistance didn't change the efficiency of the power supply at 0.449V under the same load.

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