A new approach to power stage design for supply interruption and change-over
A power source does not only supply a single load. Connecting to several subsystems and several loads is common. When one of the loads is increased or the device is applied, the bus voltage properly drops because of the sudden increasing current. Especially for loads such as motors, the voltage drop is more obvious. Generally, the current DC-DCs can cope with the input deviation of voltage drop. However, if a supply interruption situation occurs, all connected devices would shut down. The standard of EN 50155 - Railway Applications - Rolling Stock - Electronic Equipment describes two conditions: supply interruption, and change-over, in which the input voltage drops to zero for 10ms to 30ms. The interruption of the input power source will make any DC/DCs stop operating temporarily even for a short period of 10ms to 30ms, and all connected loads will lose their functions, then the equipment will face several seconds to several minutes to restart. The process of shutdown and restart may be hazardous for a moving train. Therefore, there are also clear regulations in EN 50155 that supply interruption and change-over class of S2, S3, and C1 must comply with criteria A; C2 must comply with criteria B. (Criteria A means that while the condition occurs, the device still maintains its function. Criteria B means that temporary performance degradation is allowed).
To avoid problems caused by supply interruption and change-over, additional capacitors must be installed at the input to provide energy during interruptions. Based on the actual output power and the conversion efficiency, input voltage, and under-voltage lockout threshold calculate the required capacitance of hold-up function. The capacitance may time a factor of 1.5 or take an appropriate margin to avoid hold-up function failure due to parts tolerance. Aluminum electrolytic capacitors are known as one of the vulnerable components. Regarding the harsh environment of railway applications, it is recommended to use electrolytic capacitors with the specifications of 125°C operating temperature and long life, and choose a reliable manufacturer. The large capacitance of the hold-up capacitors will relatively cause a high inrush current at the moment of power applied. It should be avoided to connect to the power bus directly. The use of a power resistor and a diode in parallel are recommended to limit and reduce the inrush current. Another challenge is the voltage specification of the capacitor is based on the different railway battery systems (24, 36, 48, 72, 96, and 110Vdc) times by 1.4 the transient voltage coefficient then takes an appropriate margin. Due to different countries applying different system voltages, different models of DC/DC converters and capacitors should be required. It takes a long time to design different power boards, and leave many different models and quantities of DC-DC modules and capacitors in the inventory.
P-DUKE has launched the QAE and HAE ultra-wide input series. They are QAE40U, QAE60U, QAE100U, HAE150U, and HAE200U, which provide from 40 to 200W output power, and the 14-160Vdc (12Vdc @ 100ms/185Vdc @ 1s) input voltage covers all standard voltages of railway applications. QAE and HAE ultra-wide input series have complete protection functions (OCP, SCP, UVP, OVP, OTP, etc.) and have certificated IEC/UL/EN 62368-1, EN 50155, EN 45545-2. The enhanced hold-up function of the HAE-U series provides a boosted charging voltage via BUS pin. This reduces the required hold-up capacitance for input voltages less than 80V. The enhanced hold-up function of QAE-U series is patented and provides a fixed charging voltage which allows a capacitor with same values of capacitance and rated voltage for all different supply voltages. With their ultra-wide input and their patented hold-up function these converters offer a single and complete solution for complying a variety of requirements in any country region for saving time and cost in the development phase.