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Power Management

energy in the clamp device with a peak power of 34.3kW. The pre-regulator circuit must be carefully designed as it will dissipate some power in normal operation and, for example, about 28W during the surge for a 15W DC/DC converter. Its output voltage should also be carefully chosen to be safe for the DC/DC converter yet high enough such that the pre-

about the same volume as the DC/DC converter (e.g. Murata UEI15 series). If 30ms interruptions are to be covered (Class C2) during a changeover from a discharged battery at 0.7 Un to a stabilised DC supply, a very expensive 1500uF 100V capacitor would be required, occupying about 17 cu cm.

An alternative approach is to use a new Figure 3: A 48V system using the RUW15 converter

regulator does not have to operate within the normal source voltage tolerances generating unnecessary dissipation. Capacitor C provides ‘hold-up’ at the input to the DC/DC converter after a supply interruption and diode D prevents the capacitor discharging back into the source. The value of C for a 15W DC/DC converter with 75% efficiency and a minimum input of 18V, for an interruption from the nominal 48V supply of 10ms (Class S2) is about 220uF, rated at 100V to withstand the highest pre-regulator output voltage with some margin (see box ‘hold-up’ calculation). This capacitor occupies about 5 cu cm. (e.g. Panasonic EEUFC2A221),

DC/DC converter is available from Murata Power Solutions, the RUW15 series, initially available with 12, 24 and 5V outputs at 15W. Its key features are an ultra-wide input range of 16 - 170V (peak), an active hold-up feature and reinforced isolation. It can be seen from Figure 1 that this range will meet the nominal and fluctuation requirements of 24V and 48V systems for EN 50155, including the 3.5 x Un surges and 24V systems for NF F 01-510. With an external pre-regulator it will also meet the requirements of 72V, 96V and 110V systems for EN 50155 and NF F 01-510 along with USA systems at nominal 37V and 74V.

Figure 4 shows an equivalent 48V nominal system to that shown in Figure 2 using the RUW15. The EMC filter remains the same but the pre-regulator is now not necessary as 3.5 x Un falls within the operating range of the converter. The ‘hold- up’ capacitor now connects to a dedicated pin on the DC/DC converter rather than on the supply and is charged up to a constant 80V approximately by an internal circuit. The capacitor automatically and seamlessly switches in to replace the supply when an interruption or brown out below the minimum operating voltage occurs. Now a capacitor of just 68uF rated at 100V is necessary for a Class S2, 10ms interruption from nominal input, about a third of the size of the first example. For Class C2 interruptions the gain is even more dramatic, with a capacitor value of only 220uF at 100V required, about a sixth of the size. In fact it is feasible to achieve this capacitance with an array of multilayer ceramic capacitors obviating the need for electrolytic types with their short life and limited reliability. An external diode D is also not required as this is incorporated within the RUW15. Note that the hold-up capacitor takes a few seconds to recharge after an interruption.

If this active hold-up feature is not

required, as may be the case with the higher input voltage ranges, it may be specified as not fitted making the converter a little more efficient in normal

operation and lower cost. A trade off with very wide input range is normally size and efficiency. The RUW maintains a creditable 75% typical at the nominal input voltages and although large by current 15W converter standards at

Picture of RUW

50mm square by 15mm high, if a pre- regulator and large electrolytic capacitor is not required, the space saving can offset the size penalty and more. Additional benefits are a ‘DC OK’ signal which gives a few milliseconds warning of the converter output dropping after an interruption longer than specified in the standard and reinforced isolation to 4kVAC. The converter is encapsulated in a metallic case with extra solder tabs for secure mechanical mounting.

Murata Power Solutions |

Paul Lee is Director of Business Development, Murata Power Solutions (UK)

Components in Electronics

December 2012/January 2013 25

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