Converter

Topics:

Introduction
Overview of the HV system
Operation of the converter
Boost converter

Introduction:
In hybrid and fully electric vehicles we find converters. The converter converts a high DC voltage into a low DC voltage. We therefore call this component a DC-DC converter. The high voltage from the HV battery of 200 to 600 volts (depending on the vehicle) is converted in the converter into 14 volts DC for the on‑board battery. The electrical components in the interior and exterior (think of the lighting, radio, door locks, electric window motors, etc.) are supplied with voltage and current by this battery.

The converter is installed in the vehicle as its own high‑voltage component. The connection for the high‑voltage cable can be recognized by the orange plastic cover.

Inside the converter there are two coils with a soft‑iron core in between. A high current flows through the coils. Due to the heat development, the converter is connected to the cooling system. The circulating coolant absorbs the heat and carries it to the radiator.

Overview of the HV system:
The high voltage from the HV battery is routed via high‑voltage cables to the inverter. In the inverter, the conversion from DC to AC takes place (the voltage inverts from direct current to alternating current). The HV electric motor (synchronous or asynchronous) is driven by this alternating voltage.

The HV battery also supplies the DC‑DC converter, which converts the high voltage into an on‑board voltage of 12 to 14 volts.

The following image shows the components of the HV system schematically.

Operation of the converter:
The converter is mounted between the HV battery and the 12‑volt on‑board battery. In the following image, the components are shown from left to right:

12‑volt on‑board battery;
capacitor (electrolytic);
interference suppression coil (to filter high‑frequency spikes);
diodes (rectifiers);
transformer with galvanically isolated windings;
H‑bridge with four transistors;
HV battery

The transfer from high voltage to 14 volts takes place by means of induction between coils. The connection between the low‑ and high‑voltage systems is galvanically isolated: this means that there is no conductive connection between the two systems.

The input winding (N2, HV side) provides a varying magnetic field in the soft‑iron core. The output winding (N1, 14‑volt side) is located in a varying magnetic field. A voltage is induced in it.

The HV system ECU drives transistors T2 and T3 into conduction (see the following image). Transistor T2 then connects the positive terminal of the HV battery to the lower side of the primary winding. The current leaves the upper side through the winding and flows back to the negative terminal of the HV battery via transistor T3.

The primary current causes a magnetic field in the transformer, which induces a voltage in the secondary winding. The induced magnetic field and therefore also the voltage in the secondary winding are lower than in the primary winding. The left‑hand battery and capacitor are charged with a DC voltage of around 14.4 volts.

The transformer only works with alternating voltages. Because batteries supply only DC voltage, a varying magnetic field is created by switching the transistors on and off.

For that reason, transistors T2 and T3 switch off, after which T1 and T4 are switched on immediately. The current in the primary winding now flows in the opposite direction (from top to bottom). This generates an opposite magnetic field in the transformer and therefore also an opposite voltage in the secondary winding. In this situation as well, the charging voltage of the battery and capacitor is around 14.4 volts.

Example:

AC in: 201.6 volts;
N1: 210 turns, R = 27.095 Ω ;
N2: 15 turns, R = 0.138 Ω;
Turn ratio (i) = N1 : N2 = 210:15 = 14;
AC out = AC in : i = 201.6 : 14 = 14.4 volts;
P in = U^2 : R = 201.6^2 : 27.095 = 1500 watts;
P out (loss‑free) = U^2 : R = 14.4 : 0.138 = 1500 watts;
Efficiency = 90%;
P out (actual) = P out * efficiency = 1500 * 0.9 = 1350 watts;
Battery current (I) = P : U = 1350 : 14.4 = 93.75 amperes.

Boost converter:
The image below shows a system overview including the boost converter and the inverter of a Toyota Prius.

The battery voltage of 201.6 volts is converted in the boost converter into a DC voltage of 650 volts. A coil and two IGBTs (transistors) are used to generate an induction voltage. In the boost converter, the reactor coil is shown between the capacitor (left) and the IGBTs T1 and T2. By continuously switching the transistors on and off, an induction voltage is generated in the reactor coil, causing the capacitor to charge.
The diode ensures that the charging voltage becomes higher and higher until the voltage reaches 650 volts.