Definition

Precharging is a step to be carried out before each system start-up. Precharging consists of limiting the voltage supply within an electrical system by means of an auxiliary supply circuit. Precharging thus enables moderate voltage to be applied without risk of damage to electrical system components.

Context

A precharge is required when a source is connected to the converter with a power switch in between and the source does not have a built-in precharge. Without a preload, the sudden closing of the switch can cause a brief current spike which in turn can damage the converter and the devices connected to it.

To limit this phenomenon, a resistor must be connected in parallel before the switch is closed. The purpose of this operation is to allow the current in the power converter to increase gradually.

Only when the voltage across the resistor (and therefore across the open switch) is less than 30V can the open switch close without risk. In fact, damage can be caused even when the converter is not switched on (CAN switched off) if a voltage is applied across its terminals. It is therefore necessary to precharge the DC-DC converter even when it is switched off.

Tame-Power’s contribution

Aware of the importance of precharging a live system, Tame-Power has specifically calculated the precharges required for its DC-DC converters. In addition, the Tame-Power HMI software has been specially designed to support the sizing of the preload resistor required for the application system.

Sizing the preload

All Tame-Power DC-DC converters require a preload prior to commissioning. Depending on the devices connected to the electrical architecture of the converter, there are different scenarios which affect the sizing of the precharge:

• Architectures that already have a precharge system
• Architectures requiring low-side precharging
• Architectures requiring high-side precharging
• Architectures requiring both low-side and high-side pre-loading

Systems with devices equipped with preload

If all the devices are equipped with integrated precharge, it is not necessary to add a precharge system to the electrical architecture.

Architectures requiring low-side or high-side precharging

When precharging is only required on the low side or high side, a precharging system must be added to the respective low side or high side terminals of the DC-DC converter. Only one additional precharge system is therefore required.

Architectures requiring low-side and high-side precharging

When precharging is required at the low side and high side terminals of the DC-DC converter, two additional precharging systems are required at the low side and high side terminals of the DC-DC converter. Two additional precharge systems are therefore required.

When precharging both the low side and the high side, it is essential to precharge the high side first.

Tame-Power solutions

Preload procedure

In order to limit the risk of damage to DC-DC converters, Tame-Power has developed a dedicated precharging sequence based on the diagram above, as follows:

1. Close SW 1 to precharge
2. Check that the voltage across the resistor is less than 30V
3. If so, close SW 2
4. Possibly open SW 1 (optional)

Preload calculations