ST AN3319 Application note

AN3319

Application note

STEVAL-ISV006V2: solar battery charger using the SPV1040

Introduction

The SPV1040 is a high efficiency, low power and low voltage DC-DC converter that provides a single output voltage up to 5.2 V. Startup is guaranteed at 0.3 V and the device operates down to 0.45 V when coming out from MPPT mode. It is a 100 kHz fixed frequency PWM step-up (or boost) converter able to maximize the energy generated by few solar cells (polycrystalline or amorphous). The duty cycle is controlled by an embedded unit running an MPPT algorithm with the goal of maximizing the power generated from the panel by continuously tracking its output voltage and current.

The SPV1040 guarantees the safety of overall application and of converter itself by stopping the PWM switching in the case of an overcurrent or overtemperature condition.

The IC integrates a 120 mΩ N-channel MOSFET power switch and a 140 mΩ P-channel MOSFET synchronous rectifier.

November 2011

Doc ID 18265 Rev 7

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www.st.com

Contents	AN3319

Contents

1	Application overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. 4
2	Boost switching application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. 5
3	SPV1040 description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	7
4	Application example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	10
5	Schematic and bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	12
6	External component selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	14
	6.1 Optional Schottky . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	18
7	Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	19
Appendix A SPV1040 parallel and series connection . . . . . . . . . . . . . . . . . . . . .		20
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		24

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AN3319	List of figures

List of figures

Figure 1. Boost application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Figure 2. PV cell curve. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Figure 3. Inductor current in continuous mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Figure 4. Inductor current in discontinuous mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Figure 5. Typical application schematic using the SPV1040 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Figure 6. SPV1040 equivalent circuit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Figure 7. MPPT working principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Figure 8. SPV1040 internal block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Figure 9. STEVAL-ISV006V2 top view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Figure 10. STEVAL-ISV006V2 bottom view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Figure 11. STEVAL-ISV006V2 schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Figure 12. STEVAL-ISV006V2 IOUT filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Figure 13. STEVAL-ISV006V2 PCB top view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Figure 14. STEVAL-ISV006V2 PCB bottom view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Figure 15. SPV1040 output parallel connection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Figure 16. SPV1040 output series connection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

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Application overview	AN3319

1 Application overview

Figure 1 shows the typical architecture of a boost converter based solar battery charger:

Figure 1. Boost application schematic

!-V

The SPV1040 adapts the characteristics of load to those of panel. In fact, a PV panel is made up of a series of PV cells. Each PV cell provides voltage and current which depend on the PV cell size, on its technology, and on the light irradiation power. The main electrical parameters of a PV panel (typically provided at light irradiation of 1000 W/m2, Tamb=25 °C) are:

●VOC (open circuit voltage)

●VMP (voltage at maximum power point)

●ISC (short-circuit current)

●IMP (current at maximum power point)

Figure 2 shows the typical characteristics of a PV cell:

Figure 2. PV cell curve

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			!-V

MPP (maximum power point) is the working point of the PV cell at which the product of the extracted voltage and current provides the maximum power.

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AN3319	Boost switching application

2 Boost switching application

A step-up (or boost) converter is a switching DC-DC converter able to generate an output voltage higher than (or at least equal to) the input voltage.

Referring to Figure 1, the switching element (Sw) is typically driven by a fixed frequency square waveform generated by a PWM controller.

When Sw is closed (ton) the inductor stores energy and its current increases with a slope depending on the voltage across the inductor and its inductance value. During this time the output voltage is sustained by COUT and the diode does not allow any charge transfer from the output to input stage.

When Sw is open (toff), the current in the inductor is forced, flowing toward the output until voltage at the input is higher than the output voltage. During this phase the current in the inductor decreases while the output voltage increases.

Figure 3 shows the behavior of inductor current.

Figure 3. Inductor current in continuous mode

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6).		,
,
,/[PLQ	(
((RQRQ	(RIIRII
WRQ	WRII			WLPH

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The energy stored in the inductor during ton is ideally equal to the energy released during toff, therefore the relation between ton and toff can be written as follows:

Dton

=-------------------------

(ton + toff)

where “D” is the duty cycle of the square waveform driving the switching element.

Boost applications can work in two different modes depending on the minimum inductor current within the switching period, that is if it is not null or null respectively:

●Continuous mode (CM)

●Discontinuous mode (DCM)

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Boost switching application	AN3319

Figure 4. Inductor current in discontinuous mode

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),XPK	6		6/54	¥6
	).			).
	,	%ON		,
		%OFF
	TON	TOFF		TIME
			TIDLE
				!-V

Obviously the efficiency is normally higher in CM.

Inductance and switching frequency (Fsw) impact the working mode. In fact, in order to have the system working in CM, the rule below should be followed:

	2		(D (1		– D))	2
L >	VOUT
	---P----IN-------		--------2-----	---F----SW-------------		-

According to the above, L is minimum for D = 50 %.

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AN3319	SPV1040 description

3 SPV1040 description

The following is a quick overview of SPV1040 functions, features, and operating modes.

Figure 5. Typical application schematic using the SPV1040

L

Lx

VOUT

RS

VBATT

RF1

VPV

XSHUT

ICTRL_PLUS

R3

GND

ICTRL_MINUS

CF RF2

R1

CIN

MPP-SET

VCTRL

COUT

DOUT

CINsns

COUTsns

R2

AM06700v1

The SPV1040 acts as an impedance adapter between the input source and output load which is:

Figure 6. SPV1040 equivalent circuit

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	9									065				5	9065
39	*/												065
								='&
3DQHO

!-V

Through the MPPT algorithm, it sets up the DC working point properly by guaranteeing ZIN = Zm (assuming Zm is the impedance of the supply source). In this way, the power extracted from the supply source (PIN = VIN * IIN) is maximum (PM = VM * IM).

The voltage-current curve shows all the available working points of the PV panel at a given solar irradiation. The voltage-power curve is derived from the voltage-current curve by plotting the product V*I for each voltage generated.

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SPV1040 description	AN3319

Figure 7. MPPT working principle

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								30$;
								3RZHU >:@
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Figure 7 shows the logical sequence followed by the device which proceeds for successive approximations in the search for the MPP. This method is called “Perturb and Observe”. The diagram shows that a comparison is made between the digital value of the power Pn generated by the solar cells and sampled at instant n, and the value acquired at the previous sampling period Pn-1. This allows the MPPT algorithm to determine the sign of duty cycle and to increment or decrement it by a predefined amount. In particular, the direction of adjustment (increment or decrement of duty cycle) remains unchanged until condition Pn≥Pn-1 occurs, that is, for as long as it registers an increase of the instantaneous power extracted from the cells string. On the contrary, when it registers a decrease of the power Pn<Pn-1, the sign of duty cycle adjustment is inverted.

In the meantime, SPV1040 sets its own duty cycle according to the MPPT algorithm, other controls are simultaneously executed in order to guarantee complete application safety. These controls are mainly implemented by integrated voltage comparators whose thresholds are properly set.

Figure 8. SPV1040 internal block diagram

Lx									VOUT
			START SIGNAL	ZERO CROSSING			-
			Vbg	DETECTOR			+		I CTRL_PLUS
	ANALOG BLOCK
			VMPP-REF					+
				OVER CURRENT
XSHUT	Ref tsruB					CONTROL
		CLOCK			DRIVERS
			OVER TEMPERATURE					-
			REVERSE POLARITY
									I CTRL_MINUS
			CLOCK	PWM				+	VMPP-REF
MPP-SET	MPP BLOCK		BURST MODE	DIGITAL
			DAC CODE	CORE				-	MPP-SET
							Iout Reg	+	VCTRL
GND							Vin Reg
							Vout Reg
								-	Vbg
									AM06703v1

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