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The Flying Battery Voltage Doubler/Split Supply Converter
Rather than a capacitor-diode voltage doubler or a flying capacitor, this topology flies the power supply
to split a single battery or single output wall wart into a dual (+ and -) power supply.



Figure 1. The LMC555 based flying battery converter doubles the
voltage and provides a common return
. It works with a battery
voltage of 1.5 volts to as close as the LMC555's maximum input voltage of
15 volts as you care to get
.



Introduction




Figure 2 Flying Battery Concept

The switch alternately grounds the positive then the negative battery terminal. When the negative battery terminal is grounded the positive terminal restores the charge in the +8.5V output capacitor through its diode. When the positive battery terminal is grounded the –8.5 volt output capacitor gets its charge restored through the other diode. Notice that the advantage of the flying battery converter over a simple half wave voltage doubler is that the output voltage is reduced by one diode drop plus the voltage loss in the active switch, where with a voltage double the loss is that of the active switch plus two diode drops. When one gets down to 5 volts or less, the additional diode drop can be a problem.

Another advantage is that the output capacitor is in parallel with the battery + switch resistance + 1 diode drop while with a voltage doubler the output capacitor is in series with the battery + switch resistance + 2 diode drops + the input capacitor.

The main drawback of the flying battery converter is that there is a P-P waveform of nearly Vin on the battery, which can result in problems because of radiation by the battery and coupling of current through stray capacitance to the battery.

A small Flying Battery converter based on a CMOS Monostable Multivibrator (LMC555)

The photograph in Figure 1 is that of thie LMC555 based flying battery converter.


Figure 3. The concept shown in Figure 2 realized using an LMC555.

The LMC555 oscillates at a nominal 47 kHz as a good trade-off between unloaded power loss and the value of the output capacitors, which are 22 uf in the schematic. 

Using a fresh 9 volt radio battery the no load current is 210 microamps. With 2k load resistors the following were obtained:

Vin 9.38V, 7.68 ma

V+ 8.40V;  I+ 4.2 ma

V- 7.92V; I- 3.91 ma

The milliamps don't quite add up, perhaps because of an instrumentation problem, but based on the number above, the efficiency is 94%.


A small Flying Battery converter based on an opamp



Figure 4. Small flying battery converter uses one or two opamps or comparators to perform the switching
The converter in this photograph uses an LM393 dual comparator to act as an oscillator that can
connect the negative battery terminal to common and an additional transistor to connect the positive
battery terminal to common.
 



Figure 5.This simple circuit used to prove the concept.


A small flying battery converter was made using 1/2 of a dual opamp. The reason I used dual opamps and comparators in these experiments is that I have a larger variety of dual opamps than single opamps.

OPAMP
V+
V-
LM358
+5.63V
-5.81
IRC2068
+5.99V
-6.46V
LF412
+6.24
-6.90
LM833
+6.36
-6.41
NE5532
+6.53
-7.05
TLC27L2
+4.11
-3.84
Table 1. Outputs for various opamps at about 4 ma. Vin = 8.99V.
The load was a red (+) or green (-) LED in series with 1k resistors.
One LED and resistor on each output.


    LOAD V+
V-
I+
I-
NONE
+8.04
-7.39
0
0
8.2K
+7.12
-6.64
869 uA
810 uA
9.2K
+7.17
-6.68
779 uA
726 uA
22K
+7.50
-6.93
341 uA
315 uA
Table 2. LM358 with Vin= 8.99V with various resistive loads.

Though the performance of the LM358 was not very good, I characterized it because it is a very common dual opamp. The dynamic impedance of the positive supply as determined by measuring the voltage while switching between the 8.2k and 9.2k loads is 556 ohms. Using the same technique the dynamic  impedance for the negative supply was found to be  8.2k and 9.2k loads is 444 ohms.

    LOAD V+
V-
I+
I-
NONE
+7.44
-8.17
0
0
8.2K
+7.11
7.64-
+866 uA
-932 uA
22k
+7.24
-7.81
+329 uA
-355 uA
Table 3. Performance of NE5532. Input voltage was 8.96 volts.

The NE55232 gave the best performance of the opamps I tried.  The dynamic impedance in the case of going from 22k to 8.2k is 242 Ohms for the positive output and 295 Ohms for the negative output.


A small Flying Battery converter based on a dual comparator

An opamp with its analog output stage does not make the best of power switch. Comparators do better. There exist comparators with output stages that can source and sink a lot of current and would probably make very good converters. But I didn't have any of them, so I made do with an LM393 which can only sink current. See Ifgure 5. 

Figure 6. The output stage of the LM393 is not able to source current.

To source current I used the second comparator in the package to drive an PN2907 PNP transistor so current from the positive battery terminal can be drawn to common (ground). A 22 ohm resistor in in sereis with the collector to keep current low enough to prevent secondary breakdown in the case of conduction overlap of the PN2907 and the transistor on the output of the LM393.



Figure 7. Circuit for LM393 comparator based higher current converter.


Loads
+ volts
- volts
No load2 +8.67 -7.78
1k loads +7.65 -7.64
Table 4. Outputs of the LM393 converter,
powered by Flying Bench Supply of 9.02 volts


Load
+volts
-volts
1k +7.33
-7.34

Table 5. Output of the LM393 converter, powered by a
9V 006P transistor radio battery that
measured 9
.022 volts.


 
A Larger Flying Converter based on CMOS



Figure 8. The flying wall wart version takes the single 12 volt
output of a wall mounted power supply and generates
±11.5 volts plus through the use of linear regulators ±9V.
 
The oscillator capacitor and the dual MOSFET are on the opposite
side of the circuit board. The two black components next to the output
connector are 1N4001 diodes to protect the regulators in case of
accidental shorting of their outputs.



In the case of the small converters, we saw that converters can be made with opamps and comparators. The best kind of switch is something designed to be a switch. That is is the idea behind the CMOS converter.
 

Figure 9. A CMOS based converter. The left-most NOR Schmitt trigger is an oscillator.

The circuit of Figure 6 is all digital CMOS. The only analog part is the left-most NOR Schmitt trigger is an oscillator that operates at a frequency close to 10 kHz. This frequency is used to keep the ripple across the output capacitors to several millivolts peak-to-peak. Resistors of 2.2 ohms are place in series with the drains of the output MOSFETs to reduce current spikes during conduction overlap of the two transistors.  As a result the MOSFETs run cool to the touch.

Before adding the 2.2 ohm resistors the unloaded MOSFETs temperature, measured on the drains, was 109° in a 28° ambient. After adding the transistors the unloaded drain leads were 31° in a 28° ambient. Clearly a faster MOSFET driver than three B-series CMOS outputs in parallel would result in cleaner switching and no need for the 2.2 ohm resistors.

The unloaded output voltages were higher than the input voltage because of ringing on the drains of the MOSFETs.

It should be noted that for very low power applications, the external MOSFETs may be omitted.

Flying Wall Wart Converter


Figure 10.  Adapted for use with a wall wart.

My intended use for the converter is as a Flying Wall Wart Converter to use on my projects. Most of my are powered by an unregulated positive voltage from a wall mounted power supply. With the additions of an LM7809 and LM7907 I was able to obtain positive and negative regulated 9 volts for use on the bench. The 1 millihenry inductors were added to the circuit to reduce the amplitude of current spikes that flowed through the equipment being powered, into the power line, and back through wall wart via its capacitive coupleing between the power line and its galvanicaly isolated output. The main idea is to reduce radiated and conducted interference and minimize noise in the outputs. The three 1 mH inductor would probably work even better if they were on a common core, such as the case of an common mode choke.

This power supply delivers ±9V at up to 60 milliamps. If not needed as a split supply, since there is no "real ground" it can deliver a regulated 18 volts.


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First posted in December, 2015

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