POWER ELECTRONICS FEATURE THE POWER BEHIND MILITARY ELECTRONICS
Advanced power conversion topologies achieve both wide input voltage range and high efficiency, allowing modular DC-DC converters to satisfy military power bus requirements. Steve Butler, director of Advanced Product Design, VPT, explains more...
M
odern military vehicles and aircraft are packed with sensitive electronics.
The electrical power in a vehicle is usually 24 VDC taken from the vehicle’s battery, while the electrical power in an aircraft could be 28 VDC, 270 VDC, or even 115 VAC - but this power bus is unregulated, noisy and subject to voltage transients. An isolated power supply is necessary to convert and regulate the many voltages required by the discrete electrical components, FPGAs, memories, and displays compiling every piece of electronic equipment. Schedule and cost pressure have driven the movement toward modular power solutions. As the trend continues toward smaller, more efficient, higher performance electronics, integrating voltage transient capability directly into the DC-DC converter modules can simplify power system design and improve overall system performance. MIL-STD-1275, currently revision E, governs 24 VDC military vehicle power. Although the voltage is taken from the vehicle’s battery, it is not a simple DC voltage. During engine starting, the voltage can dip to 12V (or 6V for revision D), and then remain at 16V during cranking. Short duration, limited energy voltage spikes, up to ±250V peak, can usually be clamped or filtered. Longer duration and higher energy surges, up to 100V, can be caused by larger switched loads or by the slow load step response of the alternator. These surges can’t be clamped, so if they don’t fall within the input range of the downstream electronics, they must be blocked with a series pass device such as a MOSFET. Military aircraft 28 VDC power is governed by MIL-STD-704, currently at revision F. The MIL standard details normal, abnormal, and emergency operation as well as electric starting. Each mode contains a steady state voltage range and possible transients. The maximum transient in revision F is 50V, while revision A includes an 80V transient. The various modes of operation and associated voltage levels from MIL-STD- 1275 and MIL-STD-704 are shown in Tables 1 and 2. Every application may not need to operate through every condition, but combining the worst case values, the total voltage variation can be as much as 6V to 100V for military vehicles or 12V to 80V for aircraft. This is a wider input
voltage range than the usual 18V to 36V of commercial or telecom derived power modules. The usual solution is additional circuitry, but this is contrary to the goal of shrinking electronics. Ideally, the DC-DC converter can handle these transient voltages directly. Most DC-DC converters use a buck
derived topology such as the forward. Although efficient, they are not suited to wide input voltage ranges, primarily due to limited conversion ratio and high voltage stress on the switches. An alternative is the flyback topology. It is very simple, with one primary switch, one secondary switch, and a single magnetic. The nD/(1-D) conversion ratio has a practical range of 0.1n to 3n, and maintains low voltage stresses on both the primary and secondary switches. While the flyback excels in a range
applications, it is often dismissed as a low power or low efficiency topology. Pitfalls include pulsating input and output current, high rms current in the output capacitor, and high output ripple. High peak and rms currents in the primary switch and output rectifier contribute to lower efficiency. Once these drawbacks are
CONDITION Steady state
Starting disturbance Cranking surge Voltage surge
Revision D starting Total range
CONDITION Normal
Steady state Transients
Abnormal Steady state Emergency
Electric starting Revision A surge
Total range / ELECTRONICS 20V to 31.5V
Overvoltage 50V, 50ms Undervoltage
0V, 7 sec
16V to 29V 12V to 29V 80V, 50ms
12V to 80V LEVEL
20V to 33V 12V for 1 sec 16V for 30 sec
100V, 50ms, 60 J 6V for 1 sec
6V to 100V LEVEL 22V to 29V
18V, 15ms 50V, 12ms
Table 2 Table 1 Figure 1: Flyback topology Figure 2: VPT’s VXR series of DC-DC convertors (left)
solved, the flyback’s benefits are realised. Pulsating output current is troublesome as power increases, but ultra-low ESR capacitors, either multi-layer ceramic or solid tantalum, are good choices for the output. These capacitors can handle high rms current reliably and provide low voltage ripple. High losses in the output rectifier are
Steve Butler, director of Advance Product Development for VPT (below)
remedied by using synchronous rectification. The output rectifier diode is replaced with a low on-resistance MOSFET which is then switched synchronously out of phase with the primary switch. The voltage drop and hence the power loss on the MOSFET can be lower than that of a Schottky rectifier. The trick is timing the gate of the MOSFET to minimize power loss. Self-drive schemes, where the gate is driven from the transformer, tend not to work well in flyback converters at high frequency. Instead, driving the MOSFETs from the PWM controller allows the precise timing necessary. At higher power levels, the flyback
topology remains a good option. Instead of beefing up the components, multiple power stages are added in parallel and operated out of phase. This enables input and output current ripple cancellation, reduces the rms current in the capacitors, and reduces ripple and filter size. Improving power conversion efficiency
not only saves energy, but by simplifying thermal design, and reducing wiring sizes, it saves weight. Clever design reduces size and improves efficiency. Integrating input voltage compliance directly into the DC- DC converter, as is achieved with VPT’s VXR series products, allows for a reduction in size and complexity and ultimately improves reliability.
VPT
www.vptpower.com ELECTRONICS | JUNE 2017 23
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