An embodiment of the invention will now be described with reference to the accompanying drawing,
A first one of the input terminals 1a is electrically coupled to a bar graph display 2. This comprises a bar graph display driver connected to an array of LEDs disposed in a row so as to represent a bar in a bar graph. The bar graph display driver is operable to light a certain quantity of the LEDs depending on the magnitude of the input voltage. For example, it may comprise ten LEDs and light each one in succession as the input voltage rises by an additional two volts.
The second input terminal 1b is connected to ground.
The first input terminal 1a is also electrically connected to a switch 4a in power conversion circuit 3. Power conversion circuit 3 comprises four switches 4a to 4d. Each switch 4a to 4d is controlled by a respective control signal on signal lines 17a to 17d generated by a control circuit 7. Switch 4a is operable to connect the input terminal 1a to the first terminal of an inductor 5. Switch 4b is operable to connect the first terminal of inductor 5 to ground. Switch 4d connects the second terminal of inductor 5 to ground and switch 4c connects the second terminal of inductor 5 to an output terminal 6a. Another output terminal 6b is provided which is connected to ground.
Typically the switches 4a to 4c will be MOSFETs, such as the Si7884DP from Vishay Semiconductors.
As already mentioned, control circuit 7 controls the switching of each of switches 4a to 4d. A suitable control circuit may be made using the LTC3780 high-efficiency, synchronous, 4-switch buck-boost controller from Linear Technology. The control circuit 7 is able to control the switches 4a to 4b in one of three modes: a buck mode when the input voltage on input terminals 1a and 1b exceeds the output voltage on output terminals 6a and 6b, a buck-boost mode when the input voltage is close in value to the output voltage and a boost mode when the input voltage is less than the output voltage. The particular manner in which control circuit 7 produces appropriate control signals on signal lines 17a to 17d is not important for an appreciation of the invention, which is concerned with how power can be supplied to control circuit 7 to ensure that these control signals are capable of causing switches 4a to 4d to switch properly.
The control circuit 7 is electrically connected, by a signal line 15, to the output voltage terminal 7a so that the output voltage can be monitored in a feedback loop. The control circuit 7 adjusts the duty cycle and sequence of switching of switches 4a to 4d so as to ensure that the output voltage remains at the desired value, irrespective of changes in the input voltage.
Typically, in a prior art circuit, the control circuit 7 would be provided with power directly from the input voltage terminals 1a and 1b. However, as has been already explained, when the input voltage on terminals 1a and 1b falls to a low value control circuit 7 will be unable to supply sufficient voltage to cause switches 4a to 4d to switch on and off properly. As a result, the switches 4a and 4d remain resistive whilst passing current and a large amount of power may be dissipated by them as a result, This can ultimately lead to their destruction.
In order to overcome this problem, a special power supply arrangement is provided for the control circuit. This comprises an auxiliary converter 8 electrically connected to the input voltage terminal la and, through diode 9, to the power supply input of control circuit 7. Auxiliary converter 8 is typically a boost converter which acts to increase the low value of voltage present at the input voltage terminals 1a and 1b to a level suitable for switching switches 4a to 4d so that control circuit 7 can fulfill its purpose. A suitable boost converter, such as the MAX669 device from MAXIM Semiconductor, can be used for auxiliary converter 8.
Auxiliary converter 8 therefore provides power, via power supply line 16, to control circuit 7 during the initial start up period of the converter. When the proper output voltage has been established on terminals 6a and 6b there is no longer any need to use auxiliary converter 8, although it could continue to be used if desired for simplicity's sake.
However, since it represents an overall inefficiency, it is desirable to shut it down and power up control circuit 7 via the output voltage instead. This is achieved by control circuit 7 asserting a signal, on signal line 14, indicating that the output voltage is at the correct value which causes auxiliary converter 8 to shut down.
The output voltage is therefore connected, via diode 11, to the control circuit 7. Diode 9 is reversed biassed and prevents any application of output voltage to the auxiliary converter 8.
Control circuit 7 however needs to be able to monitor which is the higher of the input and Output voltage so that it can switch to buck mode when the input voltage exceeds the output voltage. To do this the control circuit 7 determines which is the higher of the output voltage monitored on signal line 15 and the voltage supplied to control circuit 7 on power supply line 16. It cannot do this if it is powered by the output voltage because the voltage on both signal line 15 and power supply line 16 will be the same. To solve this problem, the input voltage is also coupled via diode 12 to control circuit 7. In this way, the higher of the output voltage and input voltage is caused to power control circuit 7 because either one of diodes 11 and 12 will be reversed biassed.
A sensor 13 is provided which monitors the input voltage and asserts a shut down signal should this fall below a preset threshold, for example 2 volts. This shut down signal prevents control circuit 7 from producing the control signals for switches 4a to 4d and thereby causes the converter to shut down. This is desirable so that the converter can be safely shut down before the input voltage drops to a low enough level where auxiliary converter 8 no longer functions properly which could lead to overheating and destruction of any of the switches 4a to 4d.
Number | Date | Country | Kind |
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0607959.4 | Apr 2006 | GB | national |