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Posted on 01 March 2019

Voltage Drops on Battery Lines, of 3ms – 6 sec, Can Cause Interrupts

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Converters 10W - 500W DC/DC avoid system interruptions

Supply line interruptions and voltage drops on battery wiring may lead to unexpected system behaviour in electronic systems.

By Willi Spiesz, Grau Elektronik, Karlsbad, Germany

 

In safety related electronic systems, like brake and door control systems used in trains or busses, system interruptions must be prevented. Supply line disturbances may have several sources, for example:

- A sudden opening, or switch bounce on closing, of the supply lines switches
- Additional switch on of large capacitive loads parallel to the voltage converter input
- A short circuit at other loads sharing the supply, during the fuse clearing time

Voltage converters must provide a very stable output voltage – not changing through input and output changes. Railway battery supply voltages may vary over a wide range – possibly fluctuating ±40% from nominal values. At a battery system nominal voltage of 72V, a dc/dc converter must operate safely from 43V to 101V input voltage. Within these limits the converter must provide a stable output voltage, for example, 12V or 24V. And output voltage must be stable during changes in load current from zero to full load. In addition, there are more dynamic deviations of the board system voltage. These may be caused by load changes on the battery, e.g. switch in of high current loads or short circuit conditions, with a common source impedance due to long line wire distances of 50m to 100m (see Figures 1 and 2).

Battery Voltage Net

Battery Voltage and Load Current

Voltage drops at the system board voltage of up to 1msec can be compensated for with the output smoothing capacitance of standard classic DC/DC converters working with switching frequencies between 25kHz and 250kHz. This is possible when allowing an output voltage drop of 5% from nominal, which the electronic load will tolerate (see Figure 3).

Input Voltage Drop

However, converters with higher switching frequencies of 400kHz to 1 MHz, (e.g. resonant converters) have only a small output capacitances of a few μF. With this small capacitance essentially no hold up time is provided. Capacitors must be connected in parallel to the load, outside of the converter if necessary.

Example:
50W converter, Vout = 12V, Iout = 4A, Cout
= 8mF (8000 μF), 5% Voltage drop.

Hold up time, as provided by the stored energy in the 8mF output capacity, can be calculated from Equation (1):
I = C dv/dt             (Equation 1)
Δt = C * Δ V / I  →  t = 8mF * (12V *.05)/4A = 1.2msec

(Neglecting an allowance for capacitor series resistance, aging, dry out of electrolytes, and line resistance).

In- Output Voltage Drop

Supply line disturbances and voltage drops, as short as 100usec, may cause the converter to switch off. Such short interruptions have no influence on the secondary electronic side. The assumption is that the electronic load can handle a voltage reduction from nominal of 5% and the DC/DC converter has enough storage capacity and no soft start or any other delaying factors when it was switched off. The reaction time to switch off the converter is typ. 0.1ms - 0.3msec. Therefore the converter does not react to smaller line disturbances. But converters are protected for longer interrupts as input currents may become excessive.

Typical voltage drops from the common source impedance of battery lines, e.g. caused through fuse blowing or crowbar protection elements like “transil” diodes blowing a fuse, can cause voltage dips with time durations between 0.1msec and 25msec.

The input operating voltage range of voltage converters is measured with under and overvoltage monitoring stages to avoid operation outside the specified input voltage range. This means, that if the battery voltage decreases to a value below the defined limit, in our example below 43V, the DC/DC converter is switched off. Otherwise the duty cycle, relation ton / switching period, for the switching power transistor and diode would become too high and eventually damage the transistor or diode. Right after switching off of the converter, the output voltage decreases. The restart of the converters or electronics can lead to a much longer system interrupt than the duration of the battery line disturbance, caused by soft start or boot procedures.

For railway systems there are different hold up times for nominal voltage and load specified as follows:

S1: no hold up time required
S2: 10ms

The railway standard EN50155 references the S2 hold up time of 10ms only to Vnominal. This can cause some problems, as high battery voltage provides longer hold up times, low battery voltage shorter hold up times.

Line voltage drops are undefined and random in occurrence and repetition rate. To avoid disturbances during whole life cycle of a voltage converter, e.g. 20 years, the hold up time design must be done careful. For example, aging of aluminium electrolytic capacitors during this period will be strongly influenced by dry out caused by heating of the electrolyte.

Input Short Circuit, Energy Direction

When relative long hold-up times are needed, energy storage cannot easily be provided by secondary output smoothing capacitors. The hold up time is then realized on the primary side of the converter - also by aluminium electrolytic capacitors, but at a higher voltage level. Grau Elektronik converters are available for hold up times, depending on output power (5W to 500W), between 10ms and 6 seconds.

Hold up time can be evaluated with the following equation:
W = ½ CV²       (Equation 2)

Referencing the example converter with Pout = 50W, with t = 10ms, leads to the capacitance value necessary:
C ≥ 2 * P / η * t / [V2² - V1²],
2 * 50W / 0.85 * 0.01s / [(72V)² - (43V)²] = 352μF

Hold up time through charging of AL capacitors on the board system nominal voltage

This equation, Equation 2, is only a rough estimation, because each DC/DC converter appears to have a negative input resistive behaviour in its current consumption. That is, high input voltage means low input current and low input voltage high current. The capacitor discharge time can be evaluated more exactly with the following equation:

V(t) = Vo * e-t/RC           (Equation 3)

Vo = charge voltage, V(t) = time dependent discharge voltage before the converter switches off, R = v(t)²/P*, C = capacity of the charging capacitor, P* = input power of the converter.

Rearanging Equation 3 and solving for t:
t = - ln (43V/72V) * ((72V/0.81A + 43V/1.36A))/2 * 660μF = 20.1ms

Therefore two capacitors, rated at 330μF / 160V, are necessary. The tolerance of the manufacturer is typically ± 20% so the worstcase scenario provides Cmin = 660μF * 0.8 = 528μF. With C = Cnominal – 20% there are still 16ms available. In meeting a 10msec hold up requirement, this allows 5.1ms for aging of the capacitors, voltage drop at the decoupling diode, as well as series resistance. The stored energy is also a function of Vin.

When the hold-up capacitor is charged only to 65V (Vin,nom – 10%), the hold up time is only:
t = 16.3ms (660μF), t = 13.1ms (528μF respectively).

This requires charging capacitors with higher voltage class than the battery line voltage. At higher voltages it is possible to store more energy in smaller volumes. Therefore, at Grau, we use a regulated auxiliary voltage that is independent from the line voltage of about 100V. The capacitances C for 10ms then becomes:
C ≥ 2 * P / η * t / [U2² - U1²], 2 * 50W / 0.85 * 0.01s / [ (100V)² - (43V)²] = 144μF.

One capacitor with C = 330μF at a voltage of 160V is enough to get a reliable hold-up time over the life of the converter.
t = - ln (43V/100V) * ((72V/0.81A + 43V/1.36A))/2 * 330μF = 16.7ms

Also, with a – 20% tolerance from the nominal capacitance, the hold up time still is:
t = - ln (43V/100V) * ((72V/0.81A + 43V/1.36A))/2 * 264μF = 13.1ms

It is necessary that the stored energy not flow back to the shorted input - otherwise there would be no advantage from the stored energy. This can be easily avoided by a series diode, for example.

For higher output power converters, 150W, 250W or 500W, energy storing with high voltage capacitors provides space and cost savings.

Both methods have their advantages and disadvantages. When charging capacitors directly from the battery line, the input current surge in a totally discharged cap must be considered - otherwise the input fuse will fail, or at least age prematurely. Charging capacitors to a higher voltage than the input line voltage means a little bit more effort in the electronics. The advantage is independence from battery variations and always the same hold up time.

Grau Elektronik converter designs have hold up circuits that are independent from input voltage fluctuations. With this concept, neither temperature nor input voltage influences hold-up time. Reliable and easily controllable hold-up times at optimized space relations are realized.

Grau Elektronik dc/dc converters, with galvanic isolation, series WBB 50W to 150W, and dc/dc converters, series DDB 250W, 500W, have efficiencies between 87% and 91% over the full input voltage range and over their rated output range of 20% to 100%.

 

 

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One Response

  1. avatar Caroline says:

    Your posntig is absolutely on the point!

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