COVER STORY
Switching & monitoring high voltage DC power supplies up to 1000V
Pinkesh Sachdev, product marketing engineer, Mixed Signal Products, Linear Technology, now part of Analog Devices, talks about high voltage power supplies and how to switch and monitor them up to 1000V
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C power supplies in the hundreds of volts are not as uncommon as one might think. An application that may first come to mind is electric vehicles where Li-Ion battery stack voltages range up to 400V. But some lesser known high voltage applications are in modern fighter aircraft, such as the F-22 Raptor and the F-35 Lightning II, which are primarily powered from 270V DC for faster and
•R), which can be used to reduce cabling weight. All of these high voltage supplies need to be switched on or off and soft-started into loads. For energy monitoring and optimisation, it is essential to digitally monitor the voltage and current flowing on these high voltage buses. Any circuit controlling these supplies needs to be galvanically isolated for operator safety and to protect the low voltage electronics from the dangerously high voltage.
losses (I2
Methods to control inrush current and monitor power
When designing high voltage supplies, an important goal is to safely control the startup inrush current into the capacitive load, e.g., the DC bus capacitors following a typical bridge rectifier. A simple method to lower inrush current is by using negative temperature coefficient (NTC) thermistors, also known as inrush current limiters (ICL, Figure 1a). These thermistors start with a high resistance (e.g., few ohms) at room temperature before the supply or load is turned on; the high resistance limits the inrush current at turn-on. As current flows, the thermistor heats up and its resistance
Figure 1a: Inrush control limiter - negative temperature coefficient (NTC) thermistor
precise performance. Large solar arrays can output 600V or higher, while rectifying AC voltages in industrial motor drives yield DC voltages ranging from 170V to 680V. And for many years, there has been research and development to move power distribution within a data centre from AC to high voltage DC (380V or ±190V), lowering power conversion steps, facility footprint and operational costs while easing integration with renewable energy such as solar.
Distributing power at higher voltages lowers current levels, reducing resistive
8 April 2017
Figure 1b: Inrush control limiter - step start relay
Components in Electronics Figure 2a: LTM9100 Anyside high voltage isolated switch controller with telemetry
drops by one to two orders of magnitude (by 10x–100x to below an ohm). These thermistors cost anywhere from $0.13 to $7 each, depending on current and resistance ratings. While simple to use, one problem is that a quick power cycling (on- off-on) may not limit inrush on the second power-up if the thermistor didn’t have sufficient time to cool down to the high resistance state. NTC thermistors suffer from wide tolerances (±25 per cent), and since the inrush current is tied to the steady state current through the resistance drop ratio, the inrush cannot be flexibly adjusted to arbitrarily low levels. ICLs find applications in vacuum cleaners, fluorescent lamps, and switched-mode power supplies, reducing inrush to the bridge rectifier’s DC bus capacitors. To overcome the NTC thermistor downside of no inrush limiting on a quick restart, a shorting relay is used in parallel with the resistor. This is known as a step start relay (Figure 1b). At turn-on, the parallel relay is open and inrush is limited by the resistor. A timer is also started; when it expires, the relay is shorted across the resistor. Load current now flows through the relay. On a quick restart, the step-start relay is able to provide inrush limiting. This technique requires the addition of a shorting relay and a timer to control its turn on. The increased
complexity raises the solution cost into the $20–$30 range. Other inrush current control techniques include zero crossing triacs, active power factor control circuits, and inductive input filtering with damping. Most of these are complicated, bulky, expensive, and applicable only to AC inputs. One method for isolated current monitoring is by using an isolation amplifier across a current sense resistor and a differential-to-single ended conversion amplifier feeding an ADC. Another method is to use an isolated delta-sigma modulator with an external digital filter.
As seen, controlling, protecting, and monitoring high voltage DC supplies requires cobbling many components together and making them work safely and seamlessly. This is not a trivial task. These discrete solutions are large, component-intensive, expensive, and lack safety certifications. An integrated and certified solution is needed to shorten design time and certification effort from many months into a few weeks.
Integrated solution for high voltage power control and telemetry The LTM9100 µModule (micromodule) IC is a compact all-in-one solution for
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