DC-Link Precharge: Resistor, NTC or Active Circuit?

An empty capacitor looks like a short circuit. If a 230 V supply is switched on near its peak (about 325 V) and the only series impedance is 0.2 Ω of wiring and ESR, the theoretical peak is over 1 600 A. In practice line inductance softens the edge, but surges of several hundred amperes are normal. They age rectifier diodes, blow fuses, trip breakers and, worst of all, weld relay contacts that are supposed to open later.

The goal of precharge is simple: hold the charging current below what the weakest part in the path can tolerate, usually the rectifier surge rating I_FSM, the fuse I²t or the contact rating of the relay, until the capacitor voltage is close to its final value.

A surprising fact drives the whole design. When a capacitor is charged from a fixed voltage through a resistor, the resistor dissipates exactly the energy that ends up in the capacitor, ½·C·V², whatever its resistance. A larger resistor gives a lower current for a longer time; the product comes out the same. With a rectified AC source the loss depends on the switch-on angle, but ½·C·V² is the usual design value.

Energy dumped into the resistor

E = ½ · C · V_dc²

Minimum resistance

R ≥ V_pk / I_max

Time constant

τ = R · C

Voltage after n·τ

V(t) = V_dc · (1 − e^(−t/τ)): 3τ → 95 %, 5τ → 99.3 %

So the resistance value is chosen from the allowed peak current, and the resistor type is chosen from the energy. These are two separate decisions, and mixing them up is the most common precharge mistake: a 10 W resistor can survive a 100 J pulse if it is built for it, and a 50 W resistor can crack if it is not.

A wirewound, ceramic or cement resistor in series with the DC link, shorted by a relay or contactor once the capacitor is charged, is the most predictable solution. The resistor sees one energy pulse per power-up and nothing during normal operation.

• Pick the resistor from its single-pulse energy or short-time overload curve, not its continuous power. Manufacturers of precharge resistors publish joules for a given pulse length; ordinary film resistors usually do not and are a poor choice.
• Leave margin for repeated cycles. If the equipment can be switched off and on again within seconds, the resistor has not cooled down, and two or three pulses can arrive back to back.
• Close the bypass after 3τ to 5τ. At 3τ the remaining voltage step is 5 % of V_dc, so the relay closes into a small current spike instead of a large one.
• Never enable the load before the bypass is closed. If the inverter starts drawing power through the precharge resistor, it dissipates load current continuously and will fail within seconds.

An NTC thermistor has a high resistance when cold and a low resistance once its own current heats it. It needs no relay and no control, which is why it appears in almost every small switch-mode power supply.

Its weakness is memory. Right after the equipment is switched off, the NTC is still hot and its resistance is still low. A quick off-on cycle, or a short mains dropout, produces almost the full uncontrolled inrush. In addition, an NTC that is not bypassed dissipates power all the time it carries load current, which costs efficiency in anything above a few hundred watts.

Rule of thumb: an NTC alone is fine for small supplies that are rarely power-cycled. For drives, chargers and anything where a fast restart is likely, use a resistor with a bypass, or an NTC with a bypass relay.

Higher-power designs often avoid the resistor entirely. A half-controlled thyristor rectifier can phase in the DC-link voltage slowly over many mains cycles. A PFC stage or a dedicated DC-DC stage can ramp the voltage under current control. Battery systems sometimes use a MOSFET in linear or PWM mode instead of a resistor. These solutions cost more parts and firmware, but they remove the energy pulse problem and allow fast, repeated restarts.

1. 1Worst-case line voltage is 400 V + 10 % = 440 V. The peak line-to-line voltage is √2 · 440 ≈ 622 V. With no load during precharge, the DC link charges almost to this peak (the familiar 1.35 · V_LL is the average under load, not the precharge end point).
2. 2The rectifier and fuses allow 30 A peak, so R ≥ 622 V / 30 A ≈ 20.7 Ω. A 22 Ω resistor is the next standard value.
3. 3With a 1 000 µF DC link, τ = 22 Ω · 1 000 µF = 22 ms. The bypass contactor can close after about 66 ms (3τ); allowing 110 ms (5τ) gives more margin.
4. 4Energy per power-up is ½ · 1 000 µF · (622 V)² ≈ 193 J. With two resistors in series, each must absorb about 97 J in a pulse of roughly 100 ms, plus margin for back-to-back restarts.

The inrush resistor calculator on this site runs the same steps and also plots the current, power and estimated temperature of the resistor over time.

In an electric vehicle the problem appears on the battery side. The inverter and on-board DC-DC converter have their own input capacitors, and closing the main contactors directly onto a 400 V or 800 V battery would weld them. A typical sequence is: close the negative main contactor, close a small precharge contactor in series with a resistor, wait until the bus voltage measured on the inverter side is close to the battery voltage, close the positive main contactor, then open the precharge contactor.

Because the source is a battery, the ½·C·V² rule applies exactly. The battery management system usually also checks that precharge finishes within an expected time window; a precharge that is too slow points to a short or a fault on the bus, and one that is too fast points to a missing capacitor or a broken sense line.

• Peak current at high line and switch-on at the voltage peak is below the rectifier I_FSM and the fuse rating.
• Resistor pulse energy rating covers ½·C·V² at high line, with margin for repeated starts.
• The bypass closes only after 3τ to 5τ, and the load is enabled only after the bypass is closed.
• There is a plan for a failed bypass: a thermal fuse, DC-link voltage monitoring, or a firmware timeout that shuts the system down.
• The behaviour after a short mains dropout has been tested, especially if an NTC is used.