TPS55340 boost circuit not making desired voltage

Question

I've created a 12 VAC -> 24 VDC boost power supply based on the TPS55340 regulator. I used a reference design from TI's WEBENCH tool, with some selected part substitutions for parts availability, coupled with a pretty standard AC->DC rectifier rated for 3 A. The load is a series/parallel array of LEDs (\$V_f = 2.7-3.7\text{ V}\$). Each series of 6 LEDs should draw 20 mA and total 240 mA.

The problem I'm having is that the voltage as measured between the +24V NET and GND is approximately 17 V, which only gives approximately 2.5 mA to each series of LEDs and makes them very dim. The datasheet (linked above) says that the output voltage is selected via the divider attached to the FB circuit and computed via $$V_{\text{out}} = 1.229\text{ V} \times \left(\frac{R_{\text{FBT}}}{R_{\text{FBB}}} + 1\right)$$

The standard example - which I happen to be using - is 187k/10k for 24 V. I did pull those two resistors off a board and measure them to verify.

Any pointers to send me on the right direction here? I'm really new to hardware design but this seems like a pretty straightforward boost circuit that I've screwed up somehow.

Critical Parts

• L1 -- SPM10040T-100M 10UH 5.8A 29.4MOHM
• L1_ALT -- RLF12560T-7R8N8R2 7.8UH 8.4A 12.24 MOHM

Entire BOM - Google Sheet

TI's Reference design

My Circuit as Built

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Answer 1

Here's a simulation with 220µF post-rectifier cap.

The current source sims a boost regulator. Since it tries to output a constant 240mA at 24V, it will input roughly constant power (neglecting efficiency) so its input current is proportional to 1/Vin. So I've used a function source, with "v(vcc)>3" to set current to zero when voltage is below 3V (I'll assume the boost just shuts down, also this avoids a division by zero) and "0.45*12/V(VCC)" to simulate the boost input current. I realized after taking the screenshot that it should be 0.5A instead of 0.45A but whatever.

As you can see the input cap is too small, so as it discharges, the boost draws even more current (green plot) until it sucks all the energy out of the cap and shuts down.

Switching to 1600µF looks a lot better, the capacitor stores enough energy to last until the next AC half-period where it gets recharged.

When the cap voltage doesn't drop too much (just a few volts) the boost input current is constant enough that you can use the back of the envelope approximation of:

\$ dv/dt = i/C \$

\$ C = \frac{ i.dt }{ dv } \$

So if you want dv = 2V drop between periods ; dt is say 8ms because even with a 50Hz AC period the rectifier should conduct 20% of the time, and i=0.5A, then this gives a required capacitance of 2000µF. 1600µF is close enough, it should work fine. 220µF is definitely not enough though.