BOOST converter help needed

[salil_desai]

i'm trying to simulate a solar charger(not in software but in actual hardware) using a DC power supply to charge a battery of 12V/7AH..for that i hav built the following BOOST circuit:
Vout = 13.8V
Iout = 0.75A
Fs = 50Khz
L = 600uH (pot core) continuous conduction mode
Cout = 100uF
Rload = 33 ohm/ 15W resistor
i'm using the voltage mode control IC 3525 to generate the PWM
and IRF540 mosfet for switching with and RCD snubber of R=10K C=270pf D=MUR860

my diode in series with the inductor is MUR860 with an RC snubber of R=20K and C=130pF


the PWM starts at approx 8.5VDC and gives a max duty of 0.45 which is enough according to my calculations..

*BOOST ACTION ACHIEVED*
NOW,from my sampling network(potential divider) i start a feed back.
my feedback is a type 3 compensation in which there is a series resistance R1= 122ohm with the inverting terminal(pin1 of 3525) of error amplifier. the NI terminal(pin2 of 3525) of E.A is given a fixed 2.5V reference..now from the o/p of E.A to the invrtng i/p of the same E.A is a series RC with R2= 193K and C1 = 330pF and a C = 22pf in parallel with this RC

NOW while testing this hardware:
NO LOAD CONDITION: i adjusted the o/p voltage Vout to 13.8V
LOAD CONDITION: as soon as i give my resistor load the o/p drops down to 12.6 and has a ripple of almost 1V...the inductor makes a hissing sound..and a current as high as 4-5A flows thru the load

please help me out here..i want stable 13.8VDC and my MAX current should be 0.75 A ..i think my feedback is not properly compensating

 

[RCinFLA]

Why do you need a boost switcher? What is intended PV panel specs?

Diode in series with inductor doesn't make sense. Are you talking about boost rectifier?

Sounds like your inductor core is saturating.

 

[salil_desai]

i need boost converter because i want to charge a 12V/7AH battery even when the solar panel voltage is as low as 8V...i step it up to 13.8 using boost converter..i'l post my ckt diagram in sometime...i dont have a soft copy of it

 

[FvM]

Your observations don't sound plausible. You're reporting a load current of 4-5A with a 33 ohm load. This would imply 120 - 150V output voltage. Very unlikely! But apart from this point, if you intend a current limited respectively constant current output, you have to design it.

 

[RCinFLA]

i need boost converter because i want to charge a 12V/7AH battery even when the solar panel voltage is as low as 8V...i step it up to 13.8 using boost converter..i'l post my ckt diagram in sometime...i dont have a soft copy of it

Solar cell is an illumination based current source that is clamped by its inherent diode. A panel for charging a 12v lead acid battery is typically 36 series connected cells.

While it is possible to use a panel with less then 24 series cells with a boost circuit to charge a 12v battery it is very unusual set up.

Maximum power point for a solar cell is where it is loaded to point where just a small amount of illumination generated current goes down inherent diode. For silicon cell this is about 0.52 vdc at 25 deg C. This has a -2 mV/deg C temp coefficient.

Any switcher driven from a variable current source must monitor the input voltage to regulate the switcher's loading so not to collapse the input voltage. As illumination level changes on PV panel the switcher must adjust its loading.

The maximum power point voltage will not change too much but the available current will drop as illumination drops. Since there is some series resistance within the PV panel the maximum power point voltage will drop slightly as PV illumination level generates more current. The largest maximum power point voltage variation is due to temperature change on PV panel. This is about -0.33% per degree C.

Capacitor at input of switcher across PV array provides the averaging on current load on PV panel. It should be able to supply at least ten cycles of inductor charging without too much loaded voltage variation. The actual capacitor value will depend on the bandwidth of the feedback loop network from input voltage sense. (which you have none in your circuit).

The feedback control system on a switcher fed from a PV panel is very different then a standard SMPS.

For MPPT tracking you have to continously adjust switcher input voltage target value to account for MPP voltage variation due to temp and voltage drop due to PV panel Rs and wiring from panel.

Ideally you optimize the V*I of the input. A simple way is to just maximized the battery charge current on output of switcher. Again this has to be continously searched as PV illumination changes.

Battery charge limits is a secondary control loop that backs off on the switcher when desired full charge voltage is reached on battery.

Again, with a proper PV panel, the switcher is usually a buck circuit. The best switcher IC to select is one that has voltage feedback control and current limit control. Use the voltage feedback for the switcher input voltage monitor. This will need invertering as you want the switcher to back off as input voltage goes lower then reference MPP voltage which is opposite the normal output voltage monitoring.

Use the current limit control for battery charge limit voltage to cause switcher to back off as output voltage to battery gets higher then target reference.

The searching algorythm is a whole new topic. This is why a processor is normally used to control the switcher with three feedback control loops. (MPP input voltage, battery voltage limit, and MPP optimization which tweeks MPP input reference voltage target).

 

[salil_desai]

Solar cell is an illumination based current source that is clamped by its inherent diode. A panel for charging a 12v lead acid battery is typically 36 series connected cells.

While it is possible to use a panel with less then 24 series cells with a boost circuit to charge a 12v battery it is very unusual set up.

Maximum power point for a solar cell is where it is loaded to point where just a small amount of illumination generated current goes down inherent diode. For silicon cell this is about 0.52 vdc at 25 deg C. This has a -2 mV/deg C temp coefficient.

Any switcher driven from a variable current source must monitor the input voltage to regulate the switcher's loading so not to collapse the input voltage. As illumination level changes on PV panel the switcher must adjust its loading.

The maximum power point voltage will not change too much but the available current will drop as illumination drops. Since there is some series resistance within the PV panel the maximum power point voltage will drop slightly as PV illumination level generates more current. The largest maximum power point voltage variation is due to temperature change on PV panel. This is about -0.33% per degree C.

Capacitor at input of switcher across PV array provides the averaging on current load on PV panel. It should be able to supply at least ten cycles of inductor charging without too much loaded voltage variation. The actual capacitor value will depend on the bandwidth of the feedback loop network from input voltage sense. (which you have none in your circuit).

The feedback control system on a switcher fed from a PV panel is very different then a standard SMPS.

For MPPT tracking you have to continously adjust switcher input voltage target value to account for MPP voltage variation due to temp and voltage drop due to PV panel Rs and wiring from panel.

Ideally you optimize the V*I of the input. A simple way is to just maximized the battery charge current on output of switcher. Again this has to be continously searched as PV illumination changes.

Battery charge limits is a secondary control loop that backs off on the switcher when desired full charge voltage is reached on battery.

Again, with a proper PV panel, the switcher is usually a buck circuit. The best switcher IC to select is one that has voltage feedback control and current limit control. Use the voltage feedback for the switcher input voltage monitor. This will need invertering as you want the switcher to back off as input voltage goes lower then reference MPP voltage which is opposite the normal output voltage monitoring.

Use the current limit control for battery charge limit voltage to cause switcher to back off as output voltage to battery gets higher then target reference.

The searching algorythm is a whole new topic. This is why a processor is normally used to control the switcher with three feedback control loops. (MPP input voltage, battery voltage limit, and MPP optimization which tweeks MPP input reference voltage target).

oh...so u mean that simulating the solar panels using a variable DC power supply is a bad idea??..can you give me some pre-designed versions so that i adjust my design accordingly..hmm..also...if we forget about the solar panels and just look at my circuit just as a boost converter ,,stil i'm faced wit h the above issues...my boost converter design is not efficient..:x..how can i get perfect at designing it??..any literature available??

 

[RCinFLA]

You can use a power supply that has a current limit adjustment. Set power supply voltage to Vmp to Voc and then use current limit adjustment to represent illumination variation between zero and Imp of panel. Put a series resistor in power supply line to represent Rs of panel.

Key is make sure switcher operates when power supply is in current limit mode where power supply is operating as high impedance source.

On your present issue, you need to understand the saturation limits of your inductor. You can look at the charging current ramping. If it starts to exponentially rise is where inductor is beginning to saturate. You must not go this high with peak current.

 

[rvionics]

Your F/B loop is unstable try using a 1~10n cap from Pin 9 of 3525 to Gnd. & check if it resolves the problem. Even if it solves your problem, remember you need to optimize the value for proper gain phase margin.

 

[lanceloare]

During the on-time of the transistor, the voltage across L is equal to Vin and the current IL increases linearly. When the transistor is turned off, the current IL flows through the diode and charges the output capacitor.

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**broken link removed**