While most parts will be chosen downstream of design decisions, several other parts will need to be chosen early and will drive other design decisions. The big parts we need to sort out first are the controller, the transformer, the MOSFET switch, and the diode.

The controller

As noted in the project brief, one reason I took up this project was to design something with the LM5155, a new boost/flyback/SEPIC controller that TI started offering in 2019. From my standpoint, it offers a number of improvements over the controller I used for a previous DCM boost controller, the LM3478. Several of the datasheet figures regarding the current-programmed mode threshold are a lot tighter on the LM5155. The soft-start feature of the LM5155 is adjustable and easy to design around. The LM5155 also supports a wider switching frequency range and very low voltage operation, though I won't be pushing either of those limits.

The transformer

Based on the formulas for the flyback converter output voltage, the turns ratio of the transformer has a big impact on the voltage conversion ratio. This is good because we can get a large conversion ratio without an excessively high duty cycle, and the LM5155 has a upper limit on the duty cycle of about 85-90%.

The transformer will also put an upper bound on the amount of output current the converter can support. From the formula on inductor current, the average inductor current is n/D' times the load current. The actual inductor current ramps up and down over the switching cycle, often significantly, so we need to pay attention to the peak current and keep the transformer core from saturating. The combination of the inductor current ripple, and the limits of the transformer will also drive the choice of a switching frequency.

There are only a few commercially available transformers that have a high turns ratio (n = 10), can support a high secondary voltage, and support decent amounts of the current. These transformers were built specifically for the LT3750 high voltage capacitor charger circuit, which is pretty close to our application, but is designed around applications where the capacitors are discharged almost instantly -- think of a camera flash. The suitable transformers I found were:

• Coilcraft DA203x series: DA2032, DA2033, DA2034
• Wurth WE-FB series: 750032050, 750032051, 750032052

For now, I want the converter to support up to a 30 mA output current, which ought to be enough to drive 10 smaller Nixie tubes with room to spare. Lower input voltage means higher currents, so the maximum current will be seen at the lowest input voltage, which I decided to put at 5 V.

Using the formulas at the end of the details section, at 5 V input and 30 mA output with n=10 turns ratio, D = 0.773 and IL = 1.32 A. If we allow up to a 50% current ripple, the peak current is 1.98 A, so for some headroom, something rated for about 3 A is good.

The DA2032 fits this spec exactly. The Wurth transformers do not list saturation current, but given the physically similar size, it's probably close to the Coilcraft part. The 750032050 has nearly identical specs to the DA2032 (price, inductance, DC resistance, and leakage inductance). The 750032051 has lower leakage inductance -- more on why this is important later -- but it's quite expensive. Unfortunately, none of these have compatible component footprints, and they are too large to place all on one board to pick and choose later, so I committed to the DA2032 since it meets the requirements and is low cost.

The MOSFET

One advantage of the flyback topology, compared to the boost converter, is that the switch doesn't need to block the full output voltage, so a lower-voltage, higher-performance part can be used. High performance, for a MOSFET, means lower on-resistance or Rds(on), and lower gate charge and capacitance (Qg). The Vds figure needs to withstand the off-state voltage of Vg + V/n, as seen in the diagram in the details section. With a maximum design input voltage of 12 V, and an output voltage of 170 V, and the 1:10 transformer, the MOSFET needs to be able to block 29 V. Some headroom is required, because between the leakage inductance of the transformer, and the output capacitance of the MOSFET, there will be ringing and overshoot when the switch turns off, unless attenuated by a snubber, and this can damage the MOSFET if the peaks exceed its Vds limit. The ringing is actually worse at lower input voltages, since it starts with the energy in the leakage inductance that is built up during the on-stage. The energy in an inductor is proportional the square of the current, which is higher at lower input voltages.

MOSFET technology continues to advance, and some of the latest and greatest switches are in Infineon's OptiMOS 6 lineup. These parts offer Rds(on) figures below 6 mΩ, and some Qg figures below 10 nC, though they are presently limited to 40 V. While it is tempting to pick the lowest Rds(on), I prefer to minimize the Qg figure at the highest Rds(on) I can tolerate, to get cleaner waveforms and have less agressive snubber circuits. For this reason, I chose the 5.9 mΩ/9.4 nC part, the BSC059N04LS6. The "BSC" is actually the larger of two packages available, but for an evaluation board, I prefer a larger part (easier to rework and probe) and there's always the option to miniaturize for a more mature board later.

The diode

While the MOSFET gets off easy, from the on-state diagram in the details section, the diode is reversed biased with a voltage of (V + n*Vg). When Vg = 12, the diode has to block 290 V! While Schottky diodes are usually the first choice in switched-mode power supplies, they are limited to lower voltages (< 200 V), so we'll need to choose a super-fast rectifier with at least a 400 V rating and deal with any consequences of diode reverse-recovery. Since the peak inductor current at the primary is about 2 A, the secondary current should not exceed 200 mA, and a 1 A rated diode should suffice. The fastest diode series I found was the ES1 series, offered by several manufacturers. The ES1G is the 400 V part, and I chose the version from ON Semi in the familiar SMA/DO-214 package.