# Inrush current limiter for a high power rectifier

I am designing a 10 kW rectifier (single phase, 230 V, 50 Hz, 44 Arms, 62 App) with power factor correction (a bridgeless boost DC-DC converter working in continuous conduction mode). All works perfectly except the enormous inrush current need to charge the capacitor (1 mF) in the first quarter cycle (>>100 A). I also want to limit capacitor charging current in the case of few cycles power lost, so I need an active solution instead of a passive one, ie. thermistor. What are the most common strategies to deal with this problem?

I am using this topology:

• Show the circuit you have including PF correction Aug 31, 2015 at 12:19
• Can't you set the PF correction circuit to do soft start? Aug 31, 2015 at 13:13
• I can't see how this configuration allow me to do soft start. At power on, capacitor acts as a short circuit, so a high current pulse will pass through diodes D3 and D4. If I turn on Q1 and Q2 it will be worse... I need to be able to control this current path. My idea is to place a MOSFET in series with Rs, buy I'm not an expert in electronics, so I need some literature to do it successfully. Aug 31, 2015 at 14:20
• How big is Rs. At powerup you shouldn't need to fire Q1 and Q2. The "inrush current" will be limited via Rs. Once you have reached a suitable link potential then enable control
– user16222
Aug 31, 2015 at 16:11
• @JonRB Rs is a shunt resistor to measure current, not a current limiting resistor. At these power levels (remember, 10 kW, 230 V, >60 A) a fixed current limiting resistor will be a hot spot. Aug 31, 2015 at 16:16

One of the most common solutions is to use a precharge resistor that is shorted out during normal operation.

You are essentially running a boost converter to facilitate power factor correction.

At 10kW you will be looking at using a SCR or an IGBT as the power device

simulate this circuit – Schematic created using CircuitLab

You would monitor the voltage across your capacitor until it reaches a level that the inrush is manageable (say ... 200V) & then gate your power device. Equally you would stop firing the power device if the voltage fell below say.. 185V.

The downside however is you have a PFC circuitry and that sort of relies on a very tight connection to the rectifier. IF it turns out this typical circuit is not appropriate the 2nd method would rely on pre-charge in the AC lines using back to back SCR. Whether you would need this 2nd method is downto specifics of your implementation. I had to use AC-side precharge with a 250kVA active rectifier.

• This solution is OK at startup time, because load can be switched off, but in the case of few cycles power lost you never will reach target voltage with a fixed resistor because load current will create a voltage drop, capacitor voltage never will reach target voltage, and inrush current problem will remains. Aug 31, 2015 at 17:01
• do you want to maintain output load during such a blackout? if so some more thought. If the output load can be disabled (due to undervoltage) then what I stated is still valid as you would use the 200V:185V hysteresis as an enabler into your load circuitry as well - why have your load drawing power when there is an outage?
– user16222
Aug 31, 2015 at 17:05
• In my region, micro (few cycles) voltage drops are common so if I am able to run the load with stored energy from the capacitor and recharge it without disconnecting the load, it will be a double victory. Aug 31, 2015 at 17:18