Hybrid bulk capacitance circuit for AC/DC charger
A circuit for a multi-voltage input AC/DC charger, such as a Universal AC input AC/DC charger, is provided, comprising a plurality of capacitors having different voltage ratings that are connected in parallel, and a switching circuit comprising input voltage sensing and comparator drive circuitry, to allow for selective connection of one or more of the plurality of capacitors, responsive to a sensed input voltage. Since bulk capacitors occupy a significant proportion of the volume of an AC/DC charger, this solution provides for a reduction in system volume, with associated improvement in the power density of an isolated AC/DC charger.
1. A bulk capacitor circuit for a Universal AC input AC/DC charger/adapter comprising:
an input for connection to a power source;
an output for connection to a DC/DC converter;
a plurality of bulk capacitors comprising:
a first bulk capacitor having capacitance value Cl and a voltage rating for a first input voltage range including a maximum rated input voltage, the first bulk capacitor being always connected;
a second bulk capacitor having a capacitance value C 2 and a voltage rating for a second input voltage range, below the first first input voltage range, and a switch to selectively connect/disconnect the second bulk capacitor;
an input voltage sensor; and
a control and driver circuitry;
wherein:
the control and driver circuitry is configured to:
receive a signal from the input voltage sensor indicative of the input AC voltage, and responsive to said signal, control the switch to connect/disconnect the second bulk capacitor;
wherein only the first bulk capacitor is connected to provide a capacitance value of Cl for the first input voltage range; and the first and second bulk capacitors are connected to provide a combined capacitance value of C 1 +C 2 for the second input voltage range; and
provide a disable/enable signal output to the DC/DC converter, whereby for the second input voltage range, the first and second bulk capacitors are connected prior to enabling the DC/DC converter.
2. The bulk capacitor circuit of claim 1 , wherein the capacitor values C 1 and C 2 are calculated based on a ripple voltage wherein:
C=2P in /(V 2 pkmin −V 2 inmin )f L
where P in is the required power, V pkmin is the required peak voltage at the minimum AC input voltage; V inmin is the required valley voltage at the minimum AC input voltage; and f L is the input line frequency.
3. The bulk capacitor circuit of claim 2 , wherein said capacitor values are selected to minimize a total capacitor volume of the plurality of capacitors.
4. The bulk capacitor circuit of claim 1 , comprising a third bulk capacitor, wherein:
only the first bulk capacitor is connected for the first input voltage range; the first and second bulk capacitors are connected for the second voltage range, and the first, second and third bulk capacitors are connected for a third input voltage range, wherein the first voltage range is higher than the second voltage range and the second voltage range higher than the third voltage range.
5. The bulk capacitor circuit of claim 1 , wherein each of said first and second bulk capacitors comprises one of a single capacitor and multiple single capacitors connected in parallel.
6. The bulk capacitor circuit of claim 1 , wherein the Universal input AC/DC charger is rated for an input of 90-265VAC and 47-63Hz, and for a power of ≤100W wherein the first input voltage range comprises 160 to 265VAC and the second input voltage range comprises 90 to 160VAC, the first bulk capacitor Cl is rated at 400V and the second bulk capacitor C 2 is rated at 250V, and where C 1 =C 2 .
7. The bulk capacitor circuit of claim 1 , wherein the DC/DC converter comprises an active clamp flyback topology or other isolated DC/DC topologies.
8. The bulk capacitor circuit of claim 1 , wherein the switch is a transistor switch in series with said second bulk capacitor.
9. A Universal AC input AC/DC switching mode power adapter comprising:
a diode bridge rectifier;
a bulk capacitance circuit;
an isolated DC/DC converter;
the bulk capacitance circuit having an input from the diode bridge rectifier and an output to isolated DC/DC converter;
the bulk capacitance circuit comprising:
a plurality of bulk capacitors comprising:
a first bulk capacitor having capacitance value C 1 and a voltage rating for a first input voltage range including a maximum rated input voltage, the first bulk capacitor being always connected;
a second bulk capacitor having a capacitance value C 2 and a voltage rating for a second input voltage range, below the first first input voltage range, and a switch to selectively connect/disconnect the second bulk capacitor;
an input voltage sensor; and
a control and driver circuitry;
wherein:
the control and driver circuitry is configured to:
receive a signal from the input voltage sensor indicative of the input AC voltage, and responsive to said signal, control the switch to connect/disconnect the second bulk capacitor;
wherein only the first bulk capacitor is connected to provide a capacitance value of C 1 for the first input voltage range; and the first and second bulk capacitors are connected to provide a combined capacitance value of C 1 +C 2 for the second input voltage range;
and
provide a disable/enable signal output to the DC/DC converter, whereby for the second input voltage range, the first and second bulk capacitors are connected prior to enabling the DC/DC converter.
10. The power adapter of claim 9 , wherein the isolated DC/DC converter has a flyback converter topology.