Power Module Engineering · Selection Guide

Every week our inbox fills up with enquiries that look almost identical. “Please quote thyristor modules, 250 amp, 1600 volt.” Simple enough. Then we ask two questions — what’s driving the load, and does the output ever need to be adjusted? — and somewhere around one enquiry in three turns out to be a diode module job. The reverse happens too. Last spring a distributor ordered diode modules for a welding rectifier retrofit. The machines ran. Nothing broke. But nobody on that shop floor could ever turn the current down, because a diode has no gate and never will.

So this is the guide we wish those buyers had in front of them. Not a textbook chapter — a shortcut through the five decisions that actually determine which part number lands on your purchase order, written by people who manufacture both.

What each module actually is

A diode module packs one or more PN junctions into a single insulated package with a copper baseplate. Forward bias it and it conducts; reverse bias it and it blocks. There is no third terminal, no gate wire, nothing to synchronize with. Most industrial types come as single dies, dual units, or six-pulse three-phase bridges, in packages that follow familiar footprints — our own MDS/MDK series and plenty of Semikron SEMIPACK-compatible types share the same hole pattern, which matters more than people expect when you’re retrofitting a drive cabinet built in 2009.

A thyristor module (SCR module) uses a four-layer PNPN structure. A pulse on the gate turns it on; it stays on until the current falls below the holding level, which in AC circuits happens naturally at every zero crossing. That latching behavior is what makes phase control possible: fire earlier in the half-cycle and more power reaches the load, fire later and less. For controlled rectifiers you’ll see single SCRs, dual anti-parallel types, half-controlled bridges (2 SCRs + 2 diodes) and full-controlled bridges. Half-controlled bridges exist for a reason — they deliver controlled DC output while keeping two diodes in the circuit to carry the freewheeling current, at a lower BOM cost than four SCRs.

There’s a third category worth naming: the thyristor-diode combo module, where an SCR and a diode share one package and one heatsink. Fewer joints, matched thermal behavior, one mounting hole less to worry about.

Side-by-side comparison

One honest note before the numbers: neither type is “better.” A diode module in an application that needs control isn’t a cost saving, it’s a missing feature. And an SCR doing a diode’s job is money and a gate driver spent for nothing.

The five parameters that pick the part number

Datasheets list dozens of parameters. In practice, five of them decide the order.

1. Voltage class — VDRM and VRRM

Take the peak of your line voltage, then add margin for switching transients and grid ugliness. The rule we apply internally:

Use the higher end for inductive loads, frequent switching, or sites with generators and arc furnaces upstream. The difference between a 1200 V and a 1600 V module is usually a few percent of the module price. A field failure is a shipment, a customs delay, and an angry phone call.

2. Continuous current — and the case temperature hiding in the footnote

IT(AV) looks like the headline number, but the rating is only valid at the case temperature printed next to it. Read that footnote. A “250 A” module rated at Tc = 85 °C is not a 250 A module in a cabinet that sits at 100 °C — it’s something meaningfully less (see the derating curve below). We’ve had more warranty arguments resolved by that single footnote than by any other clause in the datasheet.

Then apply a margin for load type:

3. Surge current — ITSM and I²t

Motors draw five to seven times rated current at start. Transformers have inrush. Short circuits happen. ITSM (10 ms, half-sine) tells you whether the die survives that first pulse; I²t tells you whether your semiconductor fuse coordinates properly. A fuse with an I²t rating above the module’s protects nothing.

4. Thermal design — where most modules actually die

The chain is simple: junction → case → heatsink → air.

Conduction losses come from P = VT0 × I(AV) + rT × I(RMS)². Get the three thermal resistances from the datasheet, measure your real cabinet temperature in summer — not the room the cabinet sits in — and leave margin. Under-torqued mounting screws quietly inflate Rth(c-s); over-torqued ones warp the baseplate and crack solder joints. Torque to spec, use thin thermal grease, and the math holds.

5. dv/dt, di/dt and the gate

A fast-rising voltage can turn an SCR on with no gate signal at all — on a three-phase bridge that means shoot-through. An RC snubber across the module (100 Ω + 100 nF is a common starting point) tames dv/dt. Fire the gate hard, at 3–5× IGT, and keep the gate leads twisted and short.

Matching modules to applications

A worked example: 90 kW soft starter on a 400 V line

Real numbers beat rules of thumb, so here’s one we ran for a pump station retrofit.

The load: 90 kW, 400 V three-phase motor, full-load current ≈ 163 A, starting current around 6× (roughly 980 A before the ramp kicks in).

Voltage: 400 V line → 566 V peak → × 2 margin → 1132 V. Take a 1600 V class module. Done.

Current: in a soft starter, six SCRs work in anti-parallel, two per phase, usually built from three dual-thyristor modules. Each SCR sees about 115 A RMS and 73 A average. With a 2× margin on the RMS value, a 250 A class module is the comfortable pick.

Surge: 980 A inrush against a module whose ITSM sits in the several-thousand-amp range. Not even close to a problem — as long as the ramp limits the current as designed.

Thermal: assume VT0 = 0.85 V and rT = 0.9 mΩ (typical for this class — check your own datasheet, seriously). Losses per SCR ≈ 0.85 × 73 + 0.0009 × 115² ≈ 74 W, call it ~148 W per module. With Rth(j-c) ≈ 0.10 K/W, a greased and properly torqued joint at 0.03 K/W, and a heatsink at 0.10 K/W:

That’s 45 degrees of headroom below the 125 °C junction limit. Now let the same cabinet run through a Mediterranean summer with dust-fouled fins — heatsink resistance creeps toward 0.20 K/W and the junction lands around 104 °C. Still alive, but the margin is thin, and that’s exactly how “it worked for two years and then died in August” happens.

The pick: a 250 A / 1600 V dual-thyristor module — our MTC-series type, footprint-compatible with Semikron SKKT 250 units. Three of them, one heatsink, an RC snubber per module, and a semiconductor fuse sized below the module’s I²t.

How these modules fail (and what it costs to prevent it)

Frequently asked questions

Can I replace a diode module with a thyristor module?

Electrically, in a controlled rectifier, yes — but it is not a drop-in swap. You’ll need a firing circuit, synchronization, and usually snubbers. If the output was fine as-is, keep the diode.

Which type is more reliable?

Diode modules, statistically. Fewer elements, no gate structure, nothing to falsely trigger. A well-cooled thyristor module still runs for decades — reliability is mostly decided by thermal design, not by the silicon type.

Are aftermarket modules really interchangeable with Semikron SEMIPACK types?

Many follow the same footprint and hole pattern, which makes them mechanically interchangeable. Electrically, compare the full datasheet — especially ITSM, Rth(j-c) and the case temperature at which IT(AV) is quoted, not just the headline current.

Why does my module keep failing at startup, but only sometimes?

Intermittent inrush above ITSM, often combined with a marginal mounting joint that worsens with thermal cycling. Check the fuse coordination and re-torque to spec.

What’s the difference between IT(AV) and IT(RMS)?

IT(AV) is the average over a half-sine conduction pattern — the number used to size heatsinks. IT(RMS) matters when the device carries AC, as in anti-parallel pairs. For a half sine wave, RMS is about 1.57× the average value.

How much voltage margin do I actually need?

2× line peak is the safe default. 1.6× works on clean grids with snubbers. Below that, you’re betting against your own utility company.

In closing

Ninety percent of module selection comes down to one question — does the output need to change? — followed by honest arithmetic on voltage, current and heat. Get those right, respect the footnote on the case temperature, and either module type will outlast the rest of the cabinet.